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
3 > three

Also, I think the passive voice ("there are 3 files") is covering up some important details. What generates the three files? Where do they come from?