Add basic CONTRIBUTING.md

This file will help people contribute to this project effectively[1]. To
kick things off, I simply borrowed some of the copy from the
CONTRIBUTING.md file in the factory_girls_rails project[2].

Closes #36.

[1]: https://github.com/blog/1184-contributing-guidelines
[2]: https://github.com/thoughtbot/factory_girl_rails/blob/master/CONTRIBUTING.md

Change-Id: Icf01c6ae8fc3c8f7b59803c540c468261578eff7