Add FAQ entry for notifier configuration

Create a new FAQ page and add a few entries related to configuring the
notifier.

Change-Id: Ibfd78b40fe65d20315ad563694dcb7c48641812c
Closes-Bug: #1422774

doc/source/FAQ.rst

@@ -0,0 +1,31 @@
+============================
+ Frequently Asked Questions
+============================
+
+I don't need notifications on the message bus. How do I disable them?
+=====================================================================
+
+Notification messages can be disabled using the ``noop`` notify
+driver. Set ``notification_driver = noop`` in your configuration file.
+
+Why does the notification publisher create queues, too? Shouldn't the subscriber do that?
+=========================================================================================
+
+The notification messages are meant to be used for integration with
+external services, including services that are not part of
+OpenStack. To ensure that the subscriber does not miss any messages if
+it starts after the publisher, ``oslo.messaging`` ensures that
+subscriber queues exist when notifications are sent.
+
+How do I change the queue names where notifications are published?
+==================================================================
+
+Notifications are published to the configured exchange using a topic
+built from a base value specified in the configuration file and the
+notification "level". The default topic is ``notifications``, so an
+info-level notification is published to the topic
+``notifications.info``. A subscriber queue of the same name is created
+automatically for each of these topics. To change the queue names,
+change the notification topic using the ``notification_topics``
+configuration option. The option accepts a list of values, so it is
+possible to publish to multiple topics.

doc/source/index.rst

@@ -22,6 +22,7 @@ Contents
    opts
    conffixture
    AMQP1.0
+   FAQ
    contributing
 
 Release Notes