Added reference to license (CC BY 4.0)

LICENSE.md

@@ -0,0 +1,5 @@
+## License
+
+This dataset is licensed under the [Creative Commons Attribution 4.0 International License (CC BY 4.0)](https://creativecommons.org/licenses/by/4.0/).
+
+You are free to share and adapt the material for any purpose, even commercially, as long as you provide appropriate attribution.