Change format of contributions document to ReST.

docs/CONTRIBUTING.md → docs/CONTRIBUTING.txt

 Welcome, and thank you for your interest in contributing!
+=========================================================
 
 If you are looking for a good way to contribute to the Cython project, please
 - have a look at the [Cython Hacker Guide](https://github.com/cython/cython/wiki/HackerGuide)