added link back to GitHub repository

Clicking around, I found myself wanting to read the actual source for the specs. Granted, there's always "View Source"; but it would seem kind of weird if the specs themselves weren't on GitHub.

Of course, they are! But you don't link to the repo anywhere (that I could see, anyway)! So I added a link.

This is what it looks like:

![add-link-to-source2](https://cloud.githubusercontent.com/assets/409328/3496794/260cdbb6-05e6-11e4-86d7-fc2fa35e3f9e.png)

(I made it green like the giant button.)