Use different ReadMe.txt files for inclusion in standard and portable release .zip files

[delphidabbler]

This issue is derived from issue #67, which has now been closed.

The proposal is to split Docs/ReadMe.txt into two separate files, one for each edition: Docs/ReadMe-standard.txt and Docs/ReadMe-portable.

Docs/ReadMe-standard.txt will be included in the install program renamed to ReadMe.txt.

Both versions of the file will be renamed by the Make release target to ReadMe.txt and included in zip files for the standard and portable editions respectively.