Merge pull request 'Document dev environment' (#11) from doc into master

Reviewed-on: https://git.p2p.legal/axiom-team/gecko/pulls/11

README.md

 # Ğecko
 
-Pay with Ğ1.
+Ğecko is a transaction client written in Dart. It is fast and secure thanks to native code compilation, Rust cryptography bindings and Duniter's new GraphQL API (GVA). It is not intended to manage member accounts, but rather simple wallets.
 
-## Getting Started
-
-Work in progress.
-<br>
--> https://forum.duniter.org/t/gecko-nouveau-client-de-paiements-1-sur-mobile-en-cours-de-developpement-dart-flutter/7857
-
-<br><br>
+The development is quite early, you can participate in the discussion [on the Duniter forum](https://forum.duniter.org/t/gecko-nouveau-client-de-paiements-1-sur-mobile-en-cours-de-developpement-dart-flutter/7857) (mostly FR)
 
+## Getting Started
 
 <div align="center">
 
@@ -19,4 +14,54 @@ Work in progress.
 
 ![Foo](https://git.p2p.legal/axiom-team/gecko/raw/commit/1cd2d63fe02949edabb69aa5fc498512c01db416/images/art/bb_gecko.png)
 
-</div>
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+</div>
+
+## Develop
+
+To contribute to the code, we advise you to install the following development environment.
+
+1. Android Studio
+  - Android VM 
+  - Android NDK
+1. Flutter SDK
+1. VSCode/Codium Flutter extension
+1. Rust toolchain
+
+This will take about 12GB on your drive and 30 min of your time (with a good connection). Don't hesitate to ask on the forum for a peer-coding session if you are stuck. 
+
+### Android Studio
+
+Android Studio will let you set up an Android VM and install tools you need.
+
+- Install [Android Studio](https://developer.android.com/studio/) using your favorite installation method.
+- At startup, do not open a project but click "configure" at the bottom of the "Welcome" menu
+- In "SDK Manager"
+    - SDK Platforms Ttab
+        - note your SDK folder location (later used for Rust environment variables)
+        - select Android 11 (R) API level 30 (default)
+    - SDK Tools
+        - select NDK (native development kit used to compile Rust to native target)
+- In "AVD Manager"
+    - create a virtual machine (ours is Pixel 4 32bits machine)
+    - launch it in the emulator
+
+If you reach this point without trouble, you're good to go for the next step.
+
+### Flutter SDK
+
+Flutter is a powerfull SDK to develop Android apps. [Install it](https://flutter.dev/docs/get-started/install/linux) with your favorite installation method.
+
+### VSCode
+
+We are using VSCode and therefore document the process for this IDE. Of course you're free to use whatever you want.
+Clone the ğecko repo and open a dart file (e.g. `lib/main.dart`). VSCode will suggest you to insall relevant extensions.
+
+### Rust toolchain
+
+After installing Rust with the standard [rustup install script](https://www.rust-lang.org/tools/install),
+follow the instructions to build Rust dependencies [in this separate README](./packages/dubp_rs/README.md).
+If you installed a 32bits VM, use the 32bits build command.
+
+### Build the app
+
+In a dart file (e.g. `lib/main.dart`), type the `F5` key to build the code. The app should open automatically in your VM which is running.
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