First pass at /community guidelines

The question has come up a few times (e.g. #12, #30 and #38), but just wanted to set some expectations on both sides about what kinds of things http://government.github.com/community/ is intended to showcase.

![screen shot 2013-10-16 at 4 44 36 pm](https://f.cloud.github.com/assets/86842/1346681/cacc18d2-36a3-11e3-844d-22ba36f8c153.png)

Feedback welcome!