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
