Update documentation
Update documentation

* Update to state that including the base class is not needed unless
  there are no pushed facts and purging of non-managed facts is still
  desired.

* Update to state it has been tested on RedHat systems as well

* Update documentation to remove the information about not having unit
  tests.

Signed-off-by: Andrew Grimberg <1f872228929e751eea07667e68fbb1655199bd12@linuxfoundation.org>

datacentred/datacentred-external_facts