Link to list of Octokit::Client instance methods in README

Fixes #523, providing a link to API methods provided by the client.

Whatcha think @ajkamel?

![screen shot 2014-10-17 at 4 53 33 pm](https://cloud.githubusercontent.com/assets/865/4686630/2119c27e-5648-11e4-94e6-16d31e8b3fd4.png)