David/translation/three first tutorial lessons

I just created the doc/en folder and added there the English translation of the first 3 french tutorial lessons. This tutorial has been a great help for me to understand how the Cesium app works and hope the translation could be helpful for other non french speaking developers. I'm not sure who to assign this merge request to, and who the reviewer should be. I hope I've done it the right way. If not, I will make the necessary corrections. I also added a link to the first translated lesson in the read me file.

doc/en/development_tutorial-01.md

+## Introduction
+
+This article is an introductory tutorial to the source code of the Cesium software. 
+
+This will allow you, through a succession of steps, to access the mastery of the tools and methods used daily by the developers of Cesium to create and modify the software.
+
+At the end of this tutorial, you will be *able to modify the software*. And if you feel like it, you can even make a change and share it with the main code repository, so that it is officially integrated and available to users!
+
+To your keyboards!
+
+## Level I: Retrieve the source code
+
+This first level consists of creating *your own version* of the software sources and retrieving this copy on your computer. You will produce:
+
+* your own account on the [GitLab de Duniter](https://git.duniter.org)
+* your own version of the software, your *fork*
+* a local copy of the source code files from your *fork*
+
+### Create a GitLab Duniter account
+
+> If you already have a GitLab account, you can skip this step.
+
+Go to [https://git.duniter.org](https://git.duniter.org/users/sign_in?redirect_to_referer=yes) (site in english).
+
+In 'Register' Fill in the 3 proposed fields:
+
+* Full name 
+* Username 
+* E-mail 
+* Password
+
+You will probably receive a confirmation email that you will need to validate. Once this step is over, you should have an account on the GitLab Duniter.
+
+### Fork the main repo
+
+Go to https://git.duniter.org/clients/cesium-grp/cesium.
+
+Click the "Fork" button at the top of the page (below the logo).
+
+### Install Git
+
+Installing Git depends on your operating system. Simply follow the instructions on : https://git-scm.com/
+
+### Clone your fork
+
+At this point, you are able to retrieve your version of the source code (your *fork*), so that you can work on it.
+
+#### Open Git from the command line
+
+To retrieve the source code, launch Git in console mode.
+
+* On Linux and MacOS, simply open Terminal
+* On Windows launch the *Git Bash* program :
+
+<img src="../fr/img/6fc638dc0a22d88da7e84dbf0371e69747767f78.png" width="432" height="80">
+
+#### Clone your fork from the command line
+
+Go back to the GitHub webpage and find the "Clone or download" button: Click on it, you can then copy the clone URL by clicking on the suitcase icon.
+
+All you have to do is go back to your Git console and enter: 
+
+    git clone <paste the copied URL>
+
+which gives **in my case**:
+
+```
+git clone git@git.duniter.org:blavenie/cesium.git
+Cloning into 'cesium'...
+ (...)
+Checking connectivity... done.
+```
+
+If you have come to a similar behavior, **great**, you now have the Cesium source code!
+ 
+## Level II: Compiling and Launching in a Browser
+
+This second level aims to obtain the basic tools to execute the source code, and verify its proper functioning. You will realize:
+
+* installation of the JavaScript runtime *Node.js*
+* the verification of the proper functioning of the source code *via* the launch of the application, in web mode.
+
+If the application launches, you will already have a fully **functional** environment!
+
+### Install Node.js
+
+#### Under Linux / MacOS
+
+Installing Node.js has become extremely simple for these OS: a tool allows you to install the version of Node.js you want, change it whenever you want and without conflict with a previous version: it is [nvm](https://github.com/creationix/nvm).
+
+You can install nvm with the following command:
+
+```bash
+curl -o- https://raw.githubusercontent.com/creationix/nvm/v0.33.6/install.sh | bash
+```
+
+Close and reopen your terminal, as shown. Then, install Node.js (choose version 12):
+
+```bash
+nvm install 12
+```
+
+You will then have the latest version of node's 5.x branch.js ready to use.
+
+##### Compilation tools
+
+Install the necessary tools for compilation.
+```bash
+sudo apt-get install build-essential
+```
+
+(mac users https://appuals.com/how-to-fix-sudo-apt-get-command-not-found-on-macos/)
+
+#### On Windows
+
+For Windows, download version 6 available from the official website of Node.js: https://nodejs.org
+
+Then launch the installer thus downloaded.
+
+### Install Cesium Node.js modules
+
+Cesium relies on third-party libraries to run called *dependencies*, such as compilation libraries (gulp, ionic, angularJS).
+
+The fact that you cloned the sources is actually not enough to launch the application. We need to get the code of the dependencies to get all the executable code of the program. To do this, go back to the Git console and move to the cloned directory:
+
+```bash
+cd cesium
+```
+
+Then, start downloading and installing the Cesium modules using the command:
+
+```bash
+npm install -g yarn gulp cordova@10.0.0 @ionic/cli web-ext
+```
+
+Then for non-global dependencies:
+
+```bash
+yarn
+```
+
+> The installation process may take several minutes. Indeed, it is necessary to download all the dependencies of Cesium and even compile some of them.
+
+If everything went well, you should get an end tree in the console, and the command prompt should have returned your hand:
+
+```bash
+yarn install v1.15.2
+[1/4] Resolving packages...
+  (...)
+$ node -e "try { require('fs').symlinkSync(require('path').resolve('node_modules/@bower_components'), 'www/lib', 'junction') } catch (e) { }"
+Done in 0.82s.
+```
+
+> You may get `npm WARN messages [...] `. Nothing serious: as the name of the message suggests, it is simply a non-blocking warning for the rest of the events.
+
+Then install the remaining dependencies (via bower):
+
+```bash
+npm run postinstall
+```
+
+### Install an IDE
+
+To develop in NodeJS, you can use the IDE of your choice:
+
+ * For example Sublime Text (not free) : https://www.sublimetext.com/
+ * Other possibilities : 
+    * VS Code (free).
+    * WebStorm (not free but very advanced operation).
+
+### Install Chrome and/or Firefox
+
+To debug Cesium javascript more easily, it is easier to use the Chrome browser
+
+## Level III: Mastering the usual commands
+
+This third level allows you to discover the few (five) commands that you will use all the time if you develop Cesium. You will learn:
+
+* configure Cesium, including the Duniter node it will use (default);
+* to launch Cesium in your browser;
+
+### Configure Cesium
+
+The default configuration of our environment is visible in the file 'app/config.json'. Several profiles are present: 'default', 'dev', etc.
+
+```json
+{
+   "default": {
+       "cacheTimeMs": 60000,
+       "fallbackLanguage": "en",
+       "rememberMe": false,
+       "showUDHistory": false,
+       "timeout": 10000,
+       "timeWarningExpireMembership": 5184000,
+       "timeWarningExpire": 7776000,
+       "useLocalStorage": true,
+       "useRelative": true,
+       "initPhase": false,
+       "expertMode": false,
+       "decimalCount": 4,
+       "helptip": {
+         "enable": true,
+         "installDocUrl": "https://github.com/duniter/duniter/blob/master/doc/install-a-node.md"
+       },
+       "node": {
+         "host": "g1.duniter.org",
+         "port": "443"
+       },
+       "plugins":{
+         "es": {
+           "enable": true,
+           "askEnable": false,
+           "host": "g1.data.duniter.fr",
+           "port": "443"
+         }
+       }
+     },
+     
+     (...)
+     "dev": {
+         "cacheTimeMs": 60000,
+         "fallbackLanguage": "fr-FR",
+         "defaultLanguage": "fr-FR",
+         "rememberMe": true,
+         "showUDHistory": false,
+         "timeout": 6000,
+         "timeWarningExpireMembership": 5184000,
+         "timeWarningExpire": 7776000,
+         "useLocalStorage": true,
+         "useRelative": true,
+         "initPhase": false,
+         "expertMode": false,
+         "decimalCount": 2,
+         "helptip": {
+           "enable": true,
+         },
+         "node": {
+           "host": "localhost",
+           "port": "9600"
+         },
+         "plugins":{
+           "es": {
+             "enable": false
+           }
+         }
+       },
+}
+```
+
+We will use the "dev" configuration, you  may use your Duniter node.
+
+Change the 'host' and 'port' values of the 'dev' configuration profile to match your Duniter node:
+
+```json
+  "dev: {
+  ...
+         "node": {
+           "host": "localhost",
+           "port": "9600"
+         },
+  ...
+```
+
+Disable the plugin "es" (used for Cesium+) :
+
+```json
+  "dev: {
+  ...
+         "plugins":{
+           "es": {
+             "enable": false
+           }
+         }
+  ...
+```
+
+To enable this configuration, now run the command:
+
+```bash
+ gulp config --env dev
+```
+
+```bash
+[17:32:34] Using gulpfile (...)/cesium/gulpfile.js
+[17:32:34] Starting 'config'...
+[17:32:34] Building `www/js/config.js` for `dev` environment...
+[17:32:36] Finished 'config' after 10 μs
+```
+
+> This command will be restarted at the time of your changes to the `app/config` file.
+
+Cesium is now configured to use your local Duniter node.
+
+### Launch Cesium (web mode)
+
+All you have to do is launch the application (in web mode) to find out if everything went well and that you are ready for the future.
+
+Run the following command :
+
+```bash
+npm start
+```
+
+ > Alternative : `ionic serve`
+
+When complete, the command displays :
+
+```bash
+Running live reload server: http://localhost:35729
+Watching: 0=www/**/*, 1=!www/lib/**/*
+Running dev server:  http://localhost:8100
+Ionic server commands, enter:
+  restart or r to restart the client app from the root
+  goto or g and a url to have the app navigate to the given url
+  consolelogs or c to enable/disable console log output
+  serverlogs or s to enable/disable server log output
+  quit or q to shutdown the server and exit
+
+ionic $ 
+```
+
+You can open a web browser at http://localhost:8100
+You should see the Cesium homepage there.
+ 
+Congratulations, you have an operational Cesium installation!
+
+### Documentation
+
+Cesium uses the Ionic framework, which has good documentation : http://ionicframework.com.
+
+Visit this site to learn more.
+
+## Level IV: Finding your way around the code
+
+### Locate software layers
+
+Open your IDE, and open the Cesium project.
+
+Search and locate in the code:
+
+* HTML templates that carry HMIs: www/templates 
+* controllers (JS): www/js/controllers 
+* services (JS): www/js/services
+
+<img src="../fr/img/a5078db3abdf71c87f245e948ce94a181b0e0f37.png" width="690" height="369">
+
+
+### Go further in the code
+
+Cesium relies on AngularJS. Excellent documentation is available on the web.
+
+__Note :__ The version of AngularJS used is a 1.x. The 2.x and above change completely and impose a complete redesign... This redesign is planned by 2019, in a version 2 of Cesium.
+
+## Level V: Debugging
+
+### On Chrome
+
+#### Open The Source Explorer
+
+Open the app in Chrome at http://localhost:8100
+
+Open the developer tools: 
+* Menu `Option > More Tools > Developer Tools` 
+* or by the keyboard shortcuts: `Ctrl + Shift + i`
+
+#### Debug a user's certification
+
+Open the source explorer, and then locate the file `dist/dist_js/app/controllers/wot-controllers.js`.
+
+Find the method `$scope.certify()`, and place a breakpoint there.
+
+Navigate the Cesium app as follows:
+
+ * Click in the menu (on the left) `Annuaire`;
+ * Searches for a user and then visualizes their identity;
+ * In `Certification received`, click on the `Certify` button;
+ * Verify that the console stops at the breakpoint.
+
+<img src="../fr/img/eca671a6d24b8e11566cfcca11b65e6c9c9c370c.png" width="690" height="223">
+
+Discover the code by scrolling down the step-by-step action.
+
+> Use the keys from `F9` to `F11`, to enter a method (F11), advance step by step (F10) or to the next breakpoint (F9), etc.
+
+
+## See more ?!
+
+You can now continue with the following levels. We will see how to modify a Cesium screen.
+
+[See more here >>](./development_tutorial-02.md)