Provide a "CONTRIBUTING.md" file to be handled by GitHub.

With a "`CONTRIBUTING`" (or "`CONTRIBUTING.md`") file in the repository, GitHub will remind prospective contributors to review the contribution guidelines for the project [1], which can potentially help keep you from having to remind those who probably haven't seen that section of the documentation before sending in their pull requests.

![image](https://f.cloud.github.com/assets/890200/581546/09a1fa56-c8a2-11e2-8601-1b5f5a19e21b.png)

[1]: [GitHub blog: "Contributing Guidelines"](http://github.com/blog/1184-contributing-guidelines "Contributing Guidelines")