Add documentation

backup_mpd_playlist.py

@@ -1,5 +1,17 @@
 #!/usr/bin/env python
-# Back up the state of MPD (current playlist and so on)
+# 
+# Back up the state of MPD (current playlist and so on); this allows you
+# to recover when you accidentally clear your playlist.
+#
+# This uses the python-mpd2 library from <https://pypi.python.org/pypi/python-mpd2>:
+# sudo pip3 install python-mpd2
+#
+# I run it from cron every 20 minutes or so (as user mpd, but you can 
+# run it as any user, passing in the backup directory with -d):
+# cat /etc/cron.d/mpd_backup
+# */20 *    * * *   mpd [ -x /var/lib/mpd/backup_mpd_playlist.py ] && python /var/lib/mpd/backup_mpd_playlist.py
+#
+# By Timothy Allen <tim@treehouse.org.za>
 
 import ast
 import datetime