also add licence info to readme

README.md

@@ -12,3 +12,19 @@ fn main() {
     println!("CO₂ readout: {} ppm", mhz19.read().unwrap());
 }
 ```
+
+## License
+
+Licensed under either of
+
+* Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or https://www.apache.org/licenses/LICENSE-2.0)
+* MIT license ([LICENSE-MIT](LICENSE-MIT) or https://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.