Link from CONTRIBUTING.md to dev onboarding (#686)

* Link from CONTRIBUTING.md to dev onboarding

* Update table of contents

CONTRIBUTING.md

@@ -3,6 +3,7 @@
 <!--- TOC -->
 
 * [Contributing code to Matrix](#contributing-code-to-matrix)
+* [Developer onboarding](#developer-onboarding)
 * [Android Studio settings](#android-studio-settings)
 * [Compilation](#compilation)
 * [Strings](#strings)
@@ -32,6 +33,10 @@ Element X Android support can be found in this room: [![Element Android Matrix r
 
 The rest of the document contains specific rules for Matrix Android projects
 
+## Developer onboarding
+
+For a detailed overview of the project, see [Developer Onboarding](./docs/_developer_onboarding.md).
+
 ## Android Studio settings
 
 Please set the "hard wrap" setting of Android Studio to 160 chars, this is the setting we use internally to format the source code (Menu `Settings/Editor/Code Style` then `Hard wrap at`).