Add some explicit documentation on how gitreceive works

    For people intending to maintain this in production, it's important
    to understand the details of how this works in case something goes
    wrong.

    To that end, I've added some explicit comments in the source code,
    and explained the key role that SSH "forced command" feature plays
    in the README.md file.
Merge pull request #15 from markstos/master

Add some explicit documentation on how gitreceive works