Add note about using the github-pages gem via pages.github.com/versions.json

... when developing a site for GitHub Pages.

![screen shot 2014-09-19 at 7 59 08 pm](https://cloud.githubusercontent.com/assets/237985/4344036/1e2d2a74-4072-11e4-9ee0-48ee444d4f18.png)
Since you're in an HTML block, I guess you'll have to put the "Gemfile" thing in a `code` block?