Add documentation for the sonos updater plugin.

docs/plugins/sonosupdate.rst

@@ -0,0 +1,23 @@
+SonosUpdate Plugin
+=================
+
+The ``sonosupdate`` plugin lets you automatically update `Sonos`_'s music
+library whenever you change your beets library.
+
+To use ``sonosupdate`` plugin, enable it in your configuration
+(see :ref:`using-plugins`).
+
+To use the ``sonosupdate`` plugin you need to install the `soco`_ library with::
+
+    pip install soco
+
+With that all in place, you'll see beets send the "update" command to your Sonos
+controller every time you change your beets library.
+
+.. Sonos: http://sonos.com/
+.. _soco: http://python-soco.com
+
+Configuration
+-------------
+
+There is nothing to configure.