Improved apt-sources README
Improved apt-sources README

Cleaning up the apt-sources README to be easier to consume. Also
removing some tripleo references from the README.

Change-Id: I6937fd5cd51288b36890dde214701bcef1d61381

takahashinobuyuki/diskimage-builder,switch-ch/diskimage-builder,zioproto/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,rdo-management/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,takahashinobuyuki/diskimage-builder,switch-ch/diskimage-builder,zioproto/diskimage-builder,openstack/diskimage-builder,openstack/diskimage-builder,rdo-management/diskimage-builder