Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Do we have a reference implementation of packing and unpacking layers? I thought there was some really basic `chroot` filesystem driver. If so, and if it is easy to understand, we should reference it from this spec as an implementation example.