Update license section in README.md

Author: bonomat

README.md

@@ -17,3 +17,20 @@ At the moment, only JsonRPC v1 APIs are supported. Support for v2 APIs may be ad
 ## Usage
 
 See `examples/`.
+
+## License
+
+Licensed under either of
+
+ * Apache License, Version 2.0
+   ([LICENSE-APACHE](LICENSE-Apache-2.0) or http://www.apache.org/licenses/LICENSE-2.0)
+ * MIT license
+   ([LICENSE-MIT](LICENSE-MIT) or 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.