Add notifiers note to install instructions

Author: rshipp

docs/installation.rst

@@ -28,3 +28,5 @@ Or if you don't know what you might need, and want to just pull in everything::
 .. note::
 
     If you'd like to use the ``git`` source, you will also need to have Git installed.
+
+    If you want to use the notifiction support, install Notifiers seperately: ``pip isntall notifiers``.