Add a reference to github_flavored_markdown in README.

As I mentioned at https://github.com/russross/blackfriday/issues/91#issuecomment-48074453, I think it's best to keep the GitHub Flavored Markdown rendering logic separate from the general, customizable parsing offered by blackfriday.

This PR adds a reference to the GFM renderer that's based on blackfriday.

It is my understanding that @russross would likely be in agreement with this approach, per https://github.com/russross/blackfriday/issues/80#issuecomment-43126205.