Add licensing text to README

Author: CAD97

README.md

@@ -8,3 +8,20 @@
 A library that provides newtype wrappers for `u32` and `(u32, u32)` for use as text offsets.
 
 See the [docs](https://docs.rs/text_unit/) for more.
+
+## 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.