Document C64 and C128 Inkwell lightpen drivers.
Removed the info about forced lightpen calibrations.

The drivers don't force them anymore.
Removed credits.txt file as is had become out-of-date.
Added port of aclock to GEOS contributed by Peter Godwin <pgodwin@gmail.com>.
Merge pull request #13 from greg-king5/gh-lightpen

Document the C64 and the C128 Inkwell lightpen drivers.
Added section about removing obsolete env vars.