README: be more explicit about the license

Simply writing "GPLv3" makes it ambiguous whether it's intended
to include the "or any later version" clause (although this was
included in the setup.py metadata).

Copy the recommended blurb from the license to make it explicit
that "any later version" does apply here.

README.md

@@ -53,4 +53,7 @@ print("Purge completed:", result)
 License
 -------
 
-GPLv3
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.