Added Gentoo Documentation

I finally got around to writing up some initial documentation/notes for installation and use of Docker on Gentoo.  As always, I'd love to hear any comments or suggestions.

I would like to eventually petition the Gentoo layman overlay maintainers to add my docker-overlay directly to the core layman repositories.xml, which would simplify one of the least well handled parts of the current install process, but I figured it'd be a good plan to get some contributed documentation for Gentoo up in the official docs next to Arch there before I did so.