Add documentation for metadata sync plugin.

docs/plugins/metasync.rst

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+MetaSync Plugin
+===============
+
+This plugin provides the ``metasync`` command, which lets you fetch certain
+metadata from other local or remote sources, for example your favorite audio
+player.
+
+Currently we support the following list of metadata sources:
+- **amarok**: This syncs rating, score, first played, last played, playcount and uid from amarok.
+
+
+Installing Dependencies
+-----------------------
+
+Fetching metadata from amarok requires the dbus-python library.
+
+There are packages for most major linux distributions, or you can download the
+library from its _website.
+
+   _website: http://dbus.freedesktop.org/releases/dbus-python/
+
+
+Configuration
+-------------
+
+To configure the plugin, make a ``metasync:`` section in your configuration
+file. The available options are:
+
+- **source**: A list of sources to fetch metadata from.
+  Default: empty
+
+
+Usage
+-----
+
+Enable the ``metasync`` plugin in your configuration (see
+:ref:`using-plugins`) then run ``beet metasync QUERY`` to fetch updated
+metadata from the configured list of sources.
+
+The command has a few command-line options:
+
+* To preview the changes that would be made without applying them, use the
+  ``-p`` (``--pretend``) flag.
+* To specify a temporary source to fetch metadata from, use the ``-s``
+  (``--source``) flag.