Add link to GitHub issues

(To encourage users to file issues.)

The link sits in the bottom of the main navigation column. It links to the issues index page to help reduce duplicate issues.

![screen shot 2014-09-21 at 22 56 26](https://cloud.githubusercontent.com/assets/111734/4350185/53661890-41da-11e4-97b7-5f71ff121d5e.png)