Move end user docs higher up in README.md.
Move end user docs higher up in README.md.

Most folk probably don't need the massive design details, so they
should come later.

Change-Id: I94067196e664b37c05fbd8882d259ffc3d95c50f

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