artifacts and docs are expected to be unique regardless of host, unlike
logs. Add a note to the README.
Change-Id: Ie66500c3e1d18147a6d86c46a6dbe9642c9fb515
When jobs put their output into output dirs, it needs to be fetched back
to the executors for publication.
Change-Id: I3dfbbd4327102b62160713ca782dfa4b13d26924