Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
"image or image layer" -- this seems odd. Do images and image layers have IDs in different namespaces?

Need the image ID necessarily be random, or is the assertion simply that it need not have semantic meaning?

If random, must the ID be from a CSPRNG?