Send email notification to user that is assigned to a change

Users want to get informed when they are assigned to a change. So far
there was no email notification on assigning a change to a user.

Change-Id: I7953b552f9814c283392b02520f5129e97cff9ff
Signed-off-by: Edwin Kempin <ekempin@google.com>
diff --git a/Documentation/config-mail.txt b/Documentation/config-mail.txt
index 63a4344..b8a2e5f 100644
--- a/Documentation/config-mail.txt
+++ b/Documentation/config-mail.txt
@@ -120,6 +120,12 @@
 change being reverted.  It is a `ChangeEmail`: see `ChangeSubject.soy` and
 ChangeFooter.
 
+=== SetAssignee.soy and SetAssigneeHtml.soy
+
+The SetAssignee templates will determine the contents of the email related to a
+user being assigned to a change. It is a `ChangeEmail`: see `ChangeSubject.soy`
+and ChangeFooter.
+
 
 == Mail Variables and Methods