Document Signed-off-by

[wking]

I added one in #4, but grepping for Signed-off-by in this repo didn't give any hits. We probably want to land some boilerplate docs around it before this repo gets too far along.

[stevvooe]

@wking Could you PR the CONTRIBUTING.md from https://github.com/docker/opensource/blob/master/project-template/CONTRIBUTING.md for me?

I am updating the deps in distribution and docker.