Add a CONTRIBUTING guide for potential contributors.

To help bugs and feature requests move faster, it's handy to answer some of the basic questions a contributor might have. Github and pengwynn/flint (a linter that checks projects for common sources of contributor friction) recommend adding a CONTRIBUTING guide. Github will even show a little message at the top of the New Issue screen if your project has one.

For more information, see [Setting guidelines for repository contributors](https://help.github.com/articles/setting-guidelines-for-repository-contributors) and [Flint](https://github.com/pengwynn/flint)

As a bit of background, when I was writing my first pull request to this project, I almost didn't notice that the repository had it's own project hooks... I just happened to test my changes inside my development environment folder and saw they had appeared in the list — I figure this might happen to other people, so I looked for a good place to put that sort of information.

This adds a simple, informal CONTRIBUTING guide with sections on writing good issues, setting up a development environment (including a reminder to `git hooks --install`) and creating pull requests... mostly I reference Github's documentation.