Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
This container config has been a strong point of confusion for me and several others. As far as I can tell:

* This provide defaults values for settings *if not specified at run time* (e.g: [`CpuShares`](https://github.com/docker/docker/blob/master/runconfig/merge.go#L20-L22))
* Some of these settings are completely ignored (e.g.: `Tty`, `Attach*`, ...)
* As far as I understand, this whole idea of "default container config" is out of [v2 image format](https://github.com/docker/docker/issues/8093) so although this is a purely v1 documentation, don't you think it might be relevant to add a "deprecation warning"?