Add Zuul ref replication

Zuul refs may now be pushed to an arbitrary number of git URLs.

Change-Id: Icae2fc94eb73b63adbf6f6b799f2be166e951b55
diff --git a/doc/source/zuul.rst b/doc/source/zuul.rst
index 0ec9f88..1a94660 100644
--- a/doc/source/zuul.rst
+++ b/doc/source/zuul.rst
@@ -160,6 +160,16 @@
   This can be overridden by individual pipelines.
   ``default_to=you@example.com``
 
+replication
+"""""""""""
+
+Zuul can push the refs it creates to any number of servers.  To do so,
+list the git push URLs in this section, one per line as follows::
+
+  [replication]
+    url1=ssh://user@host1.example.com:port/path/to/repo
+    url2=ssh://user@host2.example.com:port/path/to/repo
+
 layout.yaml
 ~~~~~~~~~~~