"Fork me on GitHub"

[jessieay]

When I first saw the guides, it was when looking at this page: https://pages.18f.gov/guides/

While the homepage has a link to the pages/guides on GitHub, there isn't a link within the guides themselves. So, when I wanted to add something, I wasn't sure where to do it.

It seems ideal to encourage contributions to the guides by making their location on GitHub very obvious. I am a fan of the "fork me on GitHub" convention as seen here: http://simonwhitaker.github.io/github-fork-ribbon-css/

Not sure what others (especially designers) think of that, but as a developer with a ❤️ for open source, I think they would be a great way to encourage contributions to these pages.

[jessieay]

closing in favor of 18F/guides#25, since that is the repo that holds the template for the guides.