Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
I began describing an `Image Name` term below `Tag` and `Repository` terms which are currently documented, but I realized that it's really difficult to begin to describe all of these things without slipping into describing what an image registry is.

I feel that the current description of `repository` and `tag` are sufficient for this spec and that domain names, ports, namespaces and their respective defaults and acceptable values are more of an expansion of `Repository` and `Tag` which should be details left for implementations to handle and document for their users.

If you disagree, please suggest a way to describe these that doesn't snowball into a full registry spec.