Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
I think it's all in [`runconfig.Merge`](https://github.com/docker/docker/blob/58ce0146e16e2e63b7a94d34a48722a9c7400c18/runconfig/merge.go#L10). Fields used:

* `Cmd`
* `CpuShares`
* `Entrypoint`
* `Env`
* `ExposedPorts` (and its legacy counterpart `PortSpecs`)
* `Memory`
* `MemorySwap`
* `User`
* `Volumes`
* `WorkingDir`