Add CONTRIBUTING.md

This document tries to explain how to file various common kinds of bug reports or enhancements.

/cc @vmg @arfon @bkeepers 
Should it be more specific about the average number of examples needed?
I found that new contributors are often only mentioning a few repositories using the new language. It doesn't really say if a language is common on GitHub and it's not always easy to search for examples when you don't know the language.