Add section in docs for unofficial bitmask support on other distros.
It would be nice to have a place where we can list external users' efforts to get bitmask working in other distros that we can't officially support right now.
e.g. https://leap.se/code/issues/3710 https://leap.se/code/issues/6233#note-13 https://github.com/leapcode/soledad/pull/187#issuecomment-65293800
(from redmine: created on 2015-05-28)