update README to document new options
update README to document new options

Change-Id: I76fe8a114459a2f72fa2d9a332c7b156e517c99d

openstack/ospurge,FlorentFlament/ospurge-packaging,DigitalPacific/ospurge,JonasPf/ospurge,openstack/ospurge,stackforge/ospurge,tnahak/ospurge,remoteur/ospurge