doc: add example calibration-helper script

This is not to be installed, except maybe as a doc. It is just an
example of what one might do.  It also has not been tested, it's just
for giving an idea of what it should do.

It also contains untested speculation.

v2:
- use syspath instead of devpath
- add license blurb

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>