Add doc page for freedesktop plugin.

docs/plugins/freedesktop.rst

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+Freedesktop Plugin
+==================
+
+The ``freedesktop`` plugin creates .directory files in your album folders. This 
+allows, when using a freedesktop.org compliant file manager, such as
+Dolphin or Nautilus, an album to have its cover art as the folder's thumbnail.
+
+To use the ``freedesktop`` plugin, enable it (see :doc:`/plugins/index`).
+
+Configuration
+-------------
+
+To configure the plugin, make a ``freedesktop:`` section in your configuration
+file. The only available option is:
+
+- ``auto``: Create .directory files automatically during import. 
+  Default: ``no``
+
+Creating .directory Files Manually
+----------------------------------
+
+The ``freedesktop`` command provided by this plugin creates .directory files
+for albums that match a query (see :doc:`/reference/query`). For example, ``beet
+freedesktop man the animal cannon`` will create the .directory file for the
+folder containing the album Man the Animal Cannon.