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
diff --git a/doc/source/launchers.rst b/doc/source/launchers.rst
index ff818be..34aa4ea 100644
--- a/doc/source/launchers.rst
+++ b/doc/source/launchers.rst
@@ -1,3 +1,4 @@
+.. _launchers:
 :title: Launchers
 
 Launchers