Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Suggested: "The filenames prefixed with `.wh.` are known as "whiteout" files."

Is the name the only indication of the special nature of these files? That is, if I had a file named `.wh.somename` actually in my tree, would the file be unpacked to the layer? I'm kind of hoping there is some permissions bit or something set that together with the name means it is a special file.