Add CONTRIBUTING docs

Including a small setup guide for flutter and android-studio
Including contribution guidelines

Author: Renarde-dev

docs/CONTRIBUTING

@@ -0,0 +1,61 @@
+# CONTRIBUTING :
+
+## Environment setup :
+
+### Flutter :
+
+Install and configure flutter using the [Official Flutter documentation](https://docs.flutter.dev/get-started/install).  
+Check `pubspec.yaml` or `.fvmrc` and change the flutter version accordingly
+
+Using git and the [commit hash of the release](https://docs.flutter.dev/release/archive)  :
+```sh
+cd <flutter path>
+git checkout d8a9f9a # Commit hash of the release
+flutter --version
+# Optional : disable analytics/telemetry :
+flutter config --no-analytics
+```
+
+Using [fvm](https://fvm.app/) :
+```sh
+fvm install 2.27.4
+cd <Project-dir>
+fvm use 2.27.4
+```
+
+### Android Studio :
+
+*(Optional - enable wayland support)* : Add `-Dawt.toolkit.name=WLToolkit` to the Custom VM Options.
+
+Install the flutter plugin and restart, then go to **Settings > Languages & Framework > Android SDK > SDK Tools** and install the following :
+
+- Android SDK Build-Tools
+- NDK
+- Android SDK Command-line Tools
+- Cmake
+- Android SDK Platform-Tools
+- Android Emulator
+
+**Enable flutter android support :**
+```sh
+flutter doctor --android-licenses # Accept the Android License
+# Check that Android toolchain is working
+flutter doctor -v
+```
+
+If Android studio doesn't pickup the path of the flutter toolchain or dart :
+
+In **Settings > Languages & Framework > Flutter** set the flutter SDK path using the absolute path of your install.  
+And in **Settings > Languages & Framework > Dart** specify the Dark SDK path `<Absolute-Flutter-Path>/bin/cache/dart-sdk`
+
+## Contributing guidelines :
+
+Contribution must target the `develop` branch and must have documented changes.
+
+if you are fixing an existing issue or bug, tag it in your merge request (example : `fixing #X - button Y not working`)
+
+if you are doing maintenance work, be sure to add the changelog of the libraries you are updating.
+
+Translation are done via [crowdin](https://crowdin.com/project/piwigo-ng)
+
+