Copy of GPL v2 license text

Downloaded from http://www.gnu.org/licenses/gpl-2.0.txt
Update README file: link to hompage, Yahoo Group; Licence now GPL v2 or later.

See http://groups.yahoo.com/neo/groups/wxAstroCapture/conversations/messages/2093
where Carsten Arnholm agrees to use the GPL v2 or later, and gives the requested
disclaimer text also added here.