Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
I wanted to differentiate the top-level fields in the Image JSON from the fields in the `config` object:

`Any extra fields in this config struct  ...` and `Any extra fields in the Image JSON struct ...` If i were to have just one, where do recommend I place it in the document?