Cleanup and clarify README

* Clarify what elastic-recheck does. Initially the primary goal was to
  report back to gerrit, but we are using elastic-recheck for much more
  then that now, so update docs to reflect that.
* Remove section on dependencies, it was used before we had a proper
  requirements file.
* Remove references to adding the resolved_at option, as that is now
  implemented

Change-Id: I6aa55bdf02174f13d86ad3309f5ad53110dc647d
Merge "Cleanup and clarify README"