GFF readme

To conform to ISO 9660 CD-ROM naming conventions, some UNIX filenames
needed to be changed.  At most FTP sites, the sao1_07.tar source file
is called saoimage.tar.Z or saoimage.1.07.tar.Z; note that the version
available via FTP is typically stored in compressed format and that
the name is subject to change as new versions become available.