[enh] Complete readme:

- add:
 - how to contribute.
 - repository content.
 - how this part of YunoHost works and interface with other YunoHost components.
 - dependencies.

README.md

@@ -1,2 +1,27 @@
-Please report issues here (no registration needed):
+# YunoHost core
-https://dev.yunohost.org/projects/yunohost/issues
+## Issues
+- [Please report issues on YunoHost bugtracker](https://dev.yunohost.org/projects/yunohost/issues) (no registration needed).
+
+## Contribute
+- You could develop on this repository using [ynh-dev tool](https://github.com/YunoHost/ynh-dev) with `use-git`  sub-command.
+- On this repository we are working [following this workflow](https://yunohost.org/#/build_system_en): `stable <— testing <— branch`.
+
+## Repository content
+- Shell [application helpers](https://yunohost.org/#/packaging_apps_helpers_en).
+- Services configuration templates.
+- Modules for the XMPP server Metronome.
+- Debian files for package creation.
+- Locales for translations of `yunohost` command.
+- YunoHost core Python 2.7 scripts.
+
+## How does it works?
+- Python core scripts are accessible through two interfaces thanks to [moulinette framework](https://github.com/YunoHost/moulinette):
+ - [CLI](https://en.wikipedia.org/wiki/Command-line_interface) for `yunohost` command.
+ - [API](https://en.wikipedia.org/wiki/Application_programming_interface) for [web administration module](https://github.com/YunoHost/yunohost-admin).
+- You could found more details about how YunoHost works on this [documentation (in french)](https://yunohost.org/#/package_list_fr).
+
+## Dependencies
+- [Python 2.7](https://www.python.org/download/releases/2.7)
+- [Moulinette](https://github.com/YunoHost/moulinette)
+- [Bash](https://www.gnu.org/software/bash/bash.html)
+- [Debian Jessie](https://www.debian.org/releases/jessie)