Add a CONTRIBUTING guide

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+# Contributing
+
+Thanks for contributing :)
+
+## Building the app
+
+The application doesn't use Gradle and it might be hard to use some features of
+Android Studio.
+
+Fortunately, there's not many dependencies:
+- OpenJDK 8
+- Android SDK: build tools `30.0.3`, platform `30`
+- Make sure to have the `$ANDROID_HOME` environment variable set.
+
+For Nix users, there's a `shell.nix` for setting-up the right environment.
+
+Building the debug apk:
+
+```sh
+make
+```
+
+If the build succeed, the debug apk is located in
+`_build/juloo.keyboard2.debug.apk`.
+
+## Debugging on your phone
+
+You need to have *USB debugging* enabled on your phone, see [Android's doc](https://developer.android.com/studio/debug/dev-options#enable).
+
+It is also possible to enable *ADB over network* in the settings and connect
+with `adb connect <ip of the phone>`. Make sure to be connected to a local
+network that you trust.
+
+Install the application with:
+
+```sh
+make installd
+```
+
+The debug version of the application won't be removed, both versions will stay
+installed at the same time.