:copyright: Mention license in README

README.md

@@ -42,3 +42,6 @@ For instance : *Q_HumainVsAlphaBeta3+.py* means that you will fight an AI using
 
 You can oppose another human being by launching the `Q_HumainVsHumain.py` python script.
 
+## License
+
+GNU GPLv3, see `LICENSE.txt`.