DOC: Adding a ``CONTRIBUTING`` document.
DOC: Adding a ``CONTRIBUTING`` document.

This will help contributors know what they need to look for when
writing code for dipy. For example, docstring, testing, and style
requirements.

FrancoisRheaultUS/dipy,FrancoisRheaultUS/dipy,matthieudumont/dipy,StongeEtienne/dipy,villalonreina/dipy,StongeEtienne/dipy,matthieudumont/dipy,nilgoyyou/dipy,villalonreina/dipy,nilgoyyou/dipy