Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
It would be worth explaining why the name of the image you cite above (`library/busybox`) and the repository name here differ.

Perhaps it's worth using an example that doesn't leave the namespace (`library/`) or the host/port (`index.provider.io:456`) implicit?