Be more explicit about the license

Stating "GPLv3" is a little too vague. In particular,
I'd like to activate the "any later version" clause,
but merely naming the license as GPLv3 makes it ambiguous
whether that was intended.

README.md

@@ -92,4 +92,7 @@ Submit pull requests against https://github.com/rohanpm/more-executors.
 
 ## 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.