Update README for neutron-fwaas-plugin
Update README for neutron-fwaas-plugin

Recent versions of fuel plugin builder creates plugins packages in RPM format,
so documentation should be updated

Change-Id: I1e335262eea5d030f75fa1290f6760dd77876b29

stackforge/fuel-plugin-neutron-fwaas,stackforge/fuel-plugin-neutron-fwaas