Document that the "Stream Events" permission is needed.

Since Gerrit 2.7 the stream-events is only available to users
that are granted the "Stream Events" permission.

Change-Id: I38960b3647145043a31e5c948141831a84643753
diff --git a/doc/source/triggers.rst b/doc/source/triggers.rst
index dd650f2..5b745e6 100644
--- a/doc/source/triggers.rst
+++ b/doc/source/triggers.rst
@@ -34,6 +34,10 @@
 be added to Gerrit.  Zuul is very flexible and can take advantage of
 those.
 
+If using Gerrit 2.7 or later, make sure the user is a member of a group
+that is granted the ``Stream Events`` permission, otherwise it will not
+be able to invoke the ``gerrit stream-events`` command over SSH.
+
 Timer
 -----