Add a little description of the image tarball format.

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)

Closes #7822
mmm, I'm adding a section that describes the tarball format that is mentioned in the sections above - I'm not sure of a good way to link them back tho.