README: extend description of ADDPACKAGES a bit
README: extend description of ADDPACKAGES a bit

Signed-off-by: Michael Adam <dcf2bf3ff8ea93047d51252815aac9756207ea09@samba.org>

obnoxxx/vagrant-lxc-base-boxes,DavBfr/vagrant-lxc-base-boxes,DavBfr/vagrant-lxc-base-boxes,sephii/vagrant-lxc-base-boxes