add license

Author: StefanoCecere

README.md

@@ -670,3 +670,7 @@ An intuitive Unity3d finite state machine (FSM). Designed with an emphasis on us
 ## Books
 
 - [Game Programming Patterns](http://gameprogrammingpatterns.com/) - Free and Paid available.
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+## License
+
+[![CC0](http://mirrors.creativecommons.org/presskit/buttons/88x31/svg/cc-zero.svg)](https://creativecommons.org/publicdomain/zero/1.0/)