Add some explicit documentation on how gitreceive works

[markstos]

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.

Thanks for this small, elegant tool!

[progrium]

This is great, thank you!