Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
oh @cpuguy83 your reworded list items seem to imply that the JSON metadata describes the container which built that layer. This is not the case. Though Docker Images do have 2 runconfig fields: `config` and `containerConfig`, your list items better describe `containerConfig` which is a field that I have removed the this specification as it is a detail of how the `docker build` subsystem works and is not necessary for describing how to run a new container from the image, rather this is the purpose of the `config` section of the JSON.