Document which Markdown engines are supported

[Blaisorblade]

I've looked at http://octopress.org/docs/setup/ to check whether Octopress supports Kramdown — in the end, I landed to imathis/octopress#1316, which belongs here. Quoting:

Octopress supports different markdown engines like kramdown or maruku . We can also change markdown itself to Git Flavoured Markdown. How about adding this to the official Documentation?

(I might submit a fix myself if I end up using octopress and learning its docs).