Add documentation about serving Zuul refs.

Change-Id: I6cdd20e6238fafdca626eb2531a3a78c985bb680
Reviewed-on: https://review.openstack.org/13974
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins
2 files changed