Document a little the concerns for operators.
Document a little the concerns for operators.

Change-Id: I2429371d3c5d931d46674c4e46d99599fda9f42e

openstack/diskimage-builder,eantyshev/diskimage-builder,NaohiroTamura/diskimage-builder,rdo-management/diskimage-builder,switch-ch/diskimage-builder,JioCloud/diskimage-builder,takahashinobuyuki/diskimage-builder,takahashinobuyuki/diskimage-builder,zioproto/diskimage-builder,NaohiroTamura/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,varunarya10/diskimage-builder,openstack/diskimage-builder,rdo-management/diskimage-builder,eantyshev/diskimage-builder,varunarya10/diskimage-builder,switch-ch/diskimage-builder,zioproto/diskimage-builder,JioCloud/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder