Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Note that PR defines CanonicalName to be LocalName, so it suggests `library/` won't appear anywhere above. Do we therefore spec `library` to be a forbidden first namespace component?

Oh, also, shall we discuss namespace and repository names and constraints here?