Add license information to readme

Author: dtolnay

README.md

@@ -2,3 +2,20 @@ Please refer to the Serde website under [**Writing a data format**] for a
 discussion of this example code.
 
 [**Writing a data format**]: https://serde.rs/data-format.html
+
+<br>
+
+#### License
+
+<sup>
+Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
+2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
+</sup>
+
+<br>
+
+<sub>
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
+be dual licensed as above, without any additional terms or conditions.
+</sub>