Add a CONTRIBUTING file

I based this off of the CONTRIBUTING.rst file I include in many of my own projects.

This is a very personal document since it sets the tone of the project for contributors.  I've imposed one tone on the document.  I suggest this be edited/rewritten in this pull request until it's appropriate for merging.

I think something resembling that third section is necessary since it's not necessarily intuitive (for newcomers) how the tests should be run at the moment.  I don't have very strong opinions on the other two sections.

django-import-export has a much more detailed CONTRIBUTING file that we could steal some ideas from: https://github.com/bmihelac/django-import-export/blob/master/CONTRIBUTING.md