Resolve "Write contributing doc"

Closes #15 (closed)

CONTRIBUTING.md

+# Contributing
+
+When contributing to this repository, please first discuss the change you wish to make via issue and
+via the [forum](https://forum.duniter.org) before making a change.
+
+Please note we have a specific workflow, please follow it in all your interactions with the project.
+
+## Workflow
+
+- You must create an issue for each feature you wish to develop, with a precise title and a
+  description of your feature. Then, assign it to yourself and click on the button
+  **`Create a merge request`**. GitLab will create a branch dedicated to the issue as well as a
+  *Work in Progress* merge request of this branch into the main branch (`dev`).
+- Please use tags to specify feature domains and concerned crates.
+- Never contribute to a branch whose issue has not been assigned to you! If the contributor make a 
+  `git rebase` your commit will be lost !
+- Before you push your commit: 
+  1.  Apply `rustfmt`. Only formatted code will be accepted, and gitlab-ci will make sure it is ok.
+      Some exceptions are accepted such as long raw strings. 
+
+      `rustfmt` is a tool applying Rust idiomatic code style to your files automatically.
+
+      If you're using rust 1.24 or greater :
+
+      ```bash
+      # Install rustfmt through rustup
+      rustup component add rustfmt
+      # Run rustfmt
+      cargo fmt
+      ```
+
+      If you're using rust 1.23 or lower :
+
+      ```bash
+      # Install nightly toolchain
+      rustup install nightly
+      # Install rustfmt through cargo
+      cargo +nightly install rustfmt
+      # Run rustfmt
+      cargo +nightly fmt
+      ```
+
+      > If you switch from 1.23 to 1.24, uninstall `rustfmt` from cargo and install it back with
+      > `rustup`
+
+  1.  Use `clippy`.
+
+      `clippy` is a linting tool scanning your code to find common mistakes or bad code.
+
+      Currenctly `clippy` is only available on the `nigthly` toolchain.
+
+      ```bash
+      # Install nightly toolchain
+      rustup install nightly
+      # Install clippy through cargo
+      cargo +nightly install clippy
+      # Run clippy
+      cargo +nightly clippy
+      ```
+  
+  1.  Document your code and avoid any unsafe feature.
+
+      Each create should contain in its root file
+
+      ```rust
+      #![deny(missing_docs, missing_debug_implementations, missing_copy_implementations,
+        trivial_casts, trivial_numeric_casts, unsafe_code, unstable_features, unused_import_braces,
+        unused_qualifications)]
+      ```
+
+      It forces you to write documentation for any public code and prevent you from using unstable
+      or unsafe code.
+
+  1.  Write unit tests, and verify that they **all** pass.
+  1.  Made a git rebase to make your history clean.
+
+      ```bash
+      # Make an alias
+      git config --global alias.tidy "rebase -i @{upstream}"
+      # Rebase your last commits since last push
+      git tidy
+      ```
+
+## Merge Process
+
+1.  Ensure any install or build dependencies are removed before the end of the layer when doing a 
+    build.
+
+1.  Ensure you rebased your branch on the latest `dev` commit to avoid any merge conflicts.
+
+1.  Update the README.md with details of changes to the interface, this includes new environment 
+    variables, exposed ports, useful file locations and container parameters.
+
+1.  Increase the version numbers in any examples files and the README.md to the new version that this
+    Pull Request would represent. The versioning scheme we use is [SemVer](http://semver.org/) :
+
+    If the create is in developement stage, its format is 0.X.Y. In production stage, it's X.Y.Z.
+
+    - When a API breaking change is made, X must be incremented.
+    - If new features are added without breaking the API, Y must be incremented.
+    - If the change is a quick fix of a production version, Z is incremented.
+
+    Your crate should always start with 0.1.0 version and only pass in 1.0.0 when no big changes are
+    planned. There is no need to rush a 1.0.0, take your time :)
+
+1.  You may merge the Merge Request in once you have the sign-off of two other developers. If you 
+    do not have permission to do that, you may request the second reviewer to merge it for you.
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