add note of licensing in README

Author: Heliozoa

README.md

@@ -96,3 +96,20 @@ A TMC plugin for Python 3.
 ### plugins/r
 
 A TMC plugin for R.
+
+## License
+
+Licensed under either of
+
+- Apache License, Version 2.0
+  ([LICENSE-APACHE](LICENSE-APACHE) 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.