Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
All the following are valid:
- `user`
- `uid`
- `user:group`
- `uid:gid`
- `uid:group`
- `user:gid`

If `group`/`gid` is not specified, the default group and supplementary groups of the given `user`/`uid` in `/etc/passwd` from the container are applied.