Add ebuild gnome-extra/polkit-gnome-0.105-r2

Autostart iif (GNOME || Unity) instead of !(MATE || KDE).
There are environments outside of MATE/KDE that also don't want GNOME stuff to
autostart.

Package-Manager: portage-2.2.8-r1
Update README.md

Add gnome-extra/polkit-gnome.