Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
good points @icecrime 

I mentioned above:

> - is every field of the `runconfig.Config` struct necessary or useful?

I can update this document to clarify this - only I'm not entirely sure which fields are ignored. I guess I can dig up the code to find out exactly what's going on: https://github.com/docker/docker/blob/58ce0146e16e2e63b7a94d34a48722a9c7400c18/daemon/daemon.go#L418

Do you happen to know which fields are used? @erikh I think you have some expertise with runconfig, could you shed any light on this?