Uses "Github flavored markdown". Specifically, code blocks are opened
using ```scheme so that Github will lex and format them as Scheme code
rather than generic monospace.
Note: I would have used ```racket, but we are still waiting for the
pygments.rb project to pull again from pygments-main -- to which I
contributed a Racket lexer back in August. After pygments.rb pulls,
can update this to use ```racket instead.
original commit: 6aa6dc0400f4fed688e6f5b5278da2e34d82ad88
