Add a timer trigger

Add a trigger that enqueues items based on a timer.

Change-Id: I024be871914c8272c237b1f818589be08cda04da
diff --git a/NEWS.rst b/NEWS.rst
index b79b4e6..f6500f0 100644
--- a/NEWS.rst
+++ b/NEWS.rst
@@ -18,11 +18,12 @@
   functionality may be achieved with a custom parameter function that
   matches all jobs).
 
-* Multiple triggers are now supported, in principle (though only
-  Gerrit is defined currently).  Your layout.yaml file will need to
+* Multiple triggers are now supported (currently Gerrit and a simple
+  Timer trigger ar supported).  Your layout.yaml file will need to
   change to add the key "gerrit:" inside of the "triggers:" list to
   specify a Gerrit trigger (and facilitate adding other kinds of
-  triggers later).  See the sample layout.yaml.
+  triggers later).  See the sample layout.yaml and Zuul section of the
+  documentation.
 
 * The default behavior is now to immediately dequeue changes that have
   merge conflicts, even those not at the head of the queue.  To enable