Frame /contribute better

Focus the message on contributing, not that we authenticate via Github.

## Before

![screen shot 2014-12-18 at 11 12 10 am](https://cloud.githubusercontent.com/assets/98233/5491507/c0e8bc12-86a7-11e4-91b6-14d50f44132b.png)

## After

![screen shot 2014-12-18 at 11 20 01 am](https://cloud.githubusercontent.com/assets/98233/5491521/d6b6b3f0-86a7-11e4-88b8-48cf9f88a888.png)