Try pressing 'm' on github
github.com
github.com
If you designed UIs assuming that that sort of user-to-user education doesn't exist, you'd be forced occasionally to reject a potentially handy power-user feature on the basis that there's no good place to promote it without cluttering the design. Which would be a shame.
http://github.github.com/github-flavored-markdown/
Markdown Cheat Sheet
On Markdown-enabled portions of the site, press M on your keyboard to display a cheat sheet.