Rename CONTRIBUTING -> CONTRIBUTING.md and README -> README.md

Adding the .md extension makes GitHub render the README and CONTRIBUTING as Markdown instead of plain text.

Before:
![screen shot 2014-07-05 at 2 42 36 pm](https://cloud.githubusercontent.com/assets/703628/3487605/7cb409bc-048d-11e4-9ce3-c7d7cdad9932.png)

After:
![screen shot 2014-07-05 at 2 43 00 pm](https://cloud.githubusercontent.com/assets/703628/3487606/7cb57ce8-048d-11e4-9a36-531930162f64.png)