Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
As it stands today, this field is omitted if there is no parent.
https://github.com/docker/docker/blob/eb3bbde6d18bb35774cd3ab2c6eba0016f679d2b/image/image.go#L26