add: license notice in README

Author: carloskiki

vrf/README.md

@@ -1 +1,17 @@
 # [RustCrypto]: Verifiable Random Functions
+
+## License
+
+Licensed under either of
+
+ * [Apache License, Version 2.0](http://www.apache.org/licenses/LICENSE-2.0)
+ * [MIT license](http://opensource.org/licenses/MIT)
+
+at your option.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.
+