Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
There's a recent Pull Request from @dkjer which has an opening which I think is an excellent description of the canonical v1 image name: https://github.com/docker/docker/pull/8456#issue-45162206