Update contribution guide to include git flow

As of yesterday we are using the git flow branching model in this project ( http://nvie.com/posts/a-successful-git-branching-model/ ). The default development branch for the next version is now `develop`, `master` only carries the status of released version (basically a history of all tags).

To use git flow you can use normal git commands or apply a number of plugins for [command line](https://github.com/nvie/gitflow), [Android Studio/IntelliJ](https://plugins.jetbrains.com/plugin/3033?pr=idea) or [SourceTree (built-in)](http://www.sourcetreeapp.com/).