Clarify whether or not future versions of the GNU GPL can be used

[Blzut3]

Original report by Jason Yundt (Bitbucket: [Jason Yundt](https://bitbucket.org/Jason Yundt), ).

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I’m going to create a Nix package for WolfstoneExtract, so I’ll need to specify which license WolfstoneExtract is available under. COPYING is a copy of the GNU GPLv3, but this repo doesn’t make it clear whether or not the code is GPL-3.0-only or GPL-3.0-or-later.

[Blzut3]

All my code is BSD so GPLv3 was chosen as the common denominator license between all the third party libraries. idCrypt’s license statement is a one line comment in the source file that does not specify “or later” so the safest assumption would be GPL-3.0-only, but this could easily be the case of the author not caring enough about legal things to know they made that implication.

You’re right that this repo could be more clear about that though.