Add start actions.

Add the ability to specify a report to gerrit on start.  This
can be used to clear the verified column.

Fixes bug #1012730.

Change-Id: I8dd2a60c3a16a8fa0046675437c750948af99577
diff --git a/doc/source/zuul.rst b/doc/source/zuul.rst
index 9ed4558..d962edb 100644
--- a/doc/source/zuul.rst
+++ b/doc/source/zuul.rst
@@ -163,6 +163,11 @@
 **failure** 
   Uses the same syntax as **success**, but describes what Zuul should
   do if at least one job fails.
+
+**start** 
+  Uses the same syntax as **success**, but describes what Zuul should
+  do when a change is added to the queue manager.  This can be used,
+  for example, to reset the value of the Verified review category.
   
 Some example queue configurations are included in the sample layout
 file.  The first is called a *check* queue::