Add "Contributing" section to README

Mostly this is to point novice contributors to a standard go-way of
contributing. Which while not perfect, is probably much better than
chaning the paths in the sources all the time.

(Which is of course doable:
 https://github.com/ericherman/code-snips/blob/master/shell/set_repo
but probably not a good idea.)

Also added a LICENSING section