Update README formatting and content
Update README formatting and content

This orders the phase subdirectories by execution order, adds
subheadings, and fixes some markdown formatting issues. This also
replaces all double-spaces after full-stops with single spaces.

Change-Id: I2d5526cbe4a902067fa9cad1456c35d13f81e183

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