Quick command reference

Linus, please pull from git-jc.git archive at:

    http://members.cox.net/junkio/git-jc.git/

(this time I have refs/heads/master there as well as HEAD; sorry but no rsync).

I've added the Documentation directory as you suggested and placed the copy of Quick reference David Greaves has been working on. I've minimally copy edited it to match the current command names after the big git-* renaming, and also added descriptions of commands and scripts that was missing back then. These three steps are recorded as separate commits (in sequence, not Octopus).

The format is still plain text. AsciiDoc can come in the next round, but let's get the ball rolling by putting something in there.

Please note that I do not intend to take the document over from David. I am just trying to help him get the result of his effort into the mainline so that the document helps wider audience, and to encourage (and remind) patch writers to keep it up to date when modifying the Plumbing.