Fix: added license info to the README

Author: stuartherbert

README.md

@@ -19,4 +19,8 @@ This extension adds one new PHP function:
     int xxhash32(string $data);
 ```
 
-It will checksum the string, and return the checksum.
+It will checksum the string, and return the checksum.
+
+## License
+
+BSD 2-clause license.