Wrote a contributing guide

[SergeStinckwich]

Since we move to github, we have to think how we manage branch management, releases/tags management and workflow to contribute. One source of inspiration could be : https://github.com/DuneSt/MaterialDesignLite/blob/development/CONTRIBUTING.md

[SergeStinckwich]

I start to wrote a contribution guide. See #57. Help is welcome !

[SergeStinckwich]

Waiting for comments.