Add contributing guidelines

I've added a [CONTRIBUTING.md](https://github.com/blog/1184-contributing-guidelines) which triggers a prompt like this when creating issues and pull requests:

![contributing](https://camo.githubusercontent.com/71995d6b0e620a9ef1ded00a04498241c69dd1bf/68747470733a2f2f6769746875622d696d616765732e73332e616d617a6f6e6177732e636f6d2f736b697463682f6973737565732d32303132303931332d3136323533392e6a7067)

This relates to #133.

Please review! :raised_hands: