Adds Docker Image v1 Spec Documention

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
It seems like any description of whiteout files and their semantics is going to #include the implementation details of a specific version of aufs, with specific config/compilation flags, along with the flags Docker invokes it with. For example, this paragraph from the [aufs documentation](http://aufs.sourceforge.net/aufs.html):

> The whiteout is for hiding files on lower branches. Also it is applied to stop readdir going lower branches. The latter case is called ’opaque directory.’ Any whiteout is an empty file, it means whiteout is just an mark. In the case of hiding lower files, the name of whiteout is ’.wh.<filename>.’ And in the case of stopping readdir, the name is ’.wh..wh..opq’ or ’.wh.__dir_opaque.’ **The name depends upon your compile configuration CONFIG_AUFS_COMPAT**. All whiteouts are hardlinked, including ’<writable branch top dir>/.wh..wh.aufs.’