Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Do you think it'd be okay to just delete the second sentence of this paragraph? I realize I probably went a little crazy in the second sentence... we really should agree on some common terminology though. It's a bit confusing to have a single term used loosely to refer to multiple things :(