Add an arbitrary number of images to this directory if you want to
use the update script and you want the background to be randomized.

update_theme.py will randomly choose one non-hidden file (i.e.
filename does not begin with a dot) and copy it to
`../background.png.`

This file is only necessary to force git to add the `backgrounds/`
directory, as adding empty directories is not possible. You can
delete this file in your local installation as long as you keep
the `backgrounds/` directory.