docs: Add documentation for the playlist plugin

docs/plugins/index.rst

@@ -81,6 +81,7 @@ like this::
    mpdupdate
    permissions
    play
+   playlist
    plexupdate
    random
    replaygain
@@ -158,6 +159,7 @@ Interoperability
 * :doc:`mpdupdate`: Automatically notifies `MPD`_ whenever the beets library
   changes.
 * :doc:`play`: Play beets queries in your music player.
+* :doc:`playlist`: Use M3U playlists tp query the beets library.
 * :doc:`plexupdate`: Automatically notifies `Plex`_ whenever the beets library
   changes.
 * :doc:`smartplaylist`: Generate smart playlists based on beets queries.

docs/plugins/playlist.rst

@@ -0,0 +1,37 @@
+Smart Playlist Plugin
+=====================
+
+``playlist`` is a plugin to use playlists in m3u format.
+
+To use it, enable the ``playlist`` plugin in your configuration
+(see :ref:`using-plugins`).
+Then configure your playlists like this::
+
+    playlist:
+        relative_to: ~/Music
+        playlist_dir: ~/.mpd/playlists
+
+It is possible to query the library based on a playlist by speicifying its
+absolute path::
+
+    $ beet ls playlist:/path/to/someplaylist.m3u
+
+The plugin also supports referencing playlists by name. The playlist is then
+seached in the playlist_dir and the ".m3u" extension is appended to the
+name::
+
+    $ beet ls playlist:anotherplaylist
+
+Configuration
+-------------
+
+To configure the plugin, make a ``smartplaylist:`` section in your
+configuration file. In addition to the ``playlists`` described above, the
+other configuration options are:
+
+- **playlist_dir**: Where to read playlist files from.
+  Default: The current working directory (i.e., ``'.'``).
+- **relative_to**: Interpret paths in the playlist files relative to a base
+  directory. It is also possible to set it to ``playlist`` to use the
+  playlist's parent directory as base directory.
+  Default: ``library``