Mention Octokit in the Libraries list

We should probably mention Octokit in the libraries list, no?

I added some styling to add a note-like callout. Let me know if you prefer it out.

![screen shot 2013-06-17 at 1 08 04 pm](https://f.cloud.github.com/assets/64050/664643/f9e77b4e-d789-11e2-976f-30cc87792921.png)