Add documentation of DIB_IMAGE_CACHE variable
Add documentation of DIB_IMAGE_CACHE variable

It is an established convention to use DIB_IMAGE_CACHE in elements to
refer to the cache location. This convention has not been documented
in the README, unlike other element development practices. This change
adds documentation for DIB_IMAGE_CACHE both from an element user and
developer perspective.

This change was prompted by I76030654e7dfc7b7d4dfa87e082c14e7cee479b9

Change-Id: I6fcc2409450cdb45e5afcb292908fbaee1af258a

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