Update apt-preferences element README from free text to table formatting
Update apt-preferences element README from free text to table formatting

Change-Id: Iae04b809c8008487fabe1a173123928ab157c78d

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