Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Sure thing! The specification was just merged in this morning! What do you think is the best way to link to it? Link to the GitHub page with the file? what if it moves in the future? should I link to the file in that specific commit?