Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
I think it's definitely worth leaving in, otherwise there is no combined format with image JSON + layer archives (as pointed out above by @metalivedev several days ago). This format is the only way to move images around other than the push/pull mechanism which uses the registry API. I don't know exactly how many there are, but there's definitely a large number of users which deal exclusively in this format in lieu of `docker push` and `docker pull`.