Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
I probably shouldn't have included the quotes in this example. I believe the way this value should be interpreted is that the substring before the first `=` is the variable name and everything after is the value. In Docker, I think this is passed directly to the execution driver in this format. @crosbymichael could you clarify this for us please?