Add documentation about gentoo overlay

user hnaparst has successfully built bitmask for gentoo.

from irc:

The install instructions are to first install the eigenlay, hnaparst, mrueg, progress, and qt overlays. Then run emerge zope.proxy bitmask_client I will have to work with the python developers to get the ebuilds reviewed and improved. Eventually, they will move first to the python overlay, and then to packages.gentoo.org. But people who are interested can run it now from my overlay.

We should add some info to the linux documentation about this overlay and follow and assist its eventual move to the officials overlays.

(from redmine: created on 2014-12-02, relates #6449 (closed))