Link to markup cheat sheet (BB-5005)

[Aryeh Leib Taurog]

In addition to the preview box (#4384), it would really help to have at least a link to a markup [[http://wikicreole.org/wiki/CheatSheet|cheat sheet]], so I don't have to go looking for it on my own, as well as a description of the markup for linking to a changeset, a user, or an issue.

See [[http://stackoverflow.com/questions/ask|stackoverflow]] for a great example of a helpful, unobtrusive set of markup cheatsheets integrated right into the editor.