contrib/udev: add note on required name for systemd

Systemd needs the file to be numbered the right way, document this
properly.

See also:
https://lists.debian.org/debian-devel-announce/2016/11/msg00008.html
http://openocd.zylin.com/2804

This fixes parts of bug #665.

Signed-off-by: Karl Palsson <karlp@tweak.net.au>
1 file changed