Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
I'd rm the bit about whiteout files (cover it later) and phrase like:

> An archive of the files which have been added, changed, or deleted in an image layer.  Using a layer-based or union filesystem such as AUFS, or by computing the diff from filesystem snapshots, the filesystem changeset can be used to present a series of image layers as if it was one cohesive filesystem.