Mention dual licensing in readme

README.md

@@ -132,6 +132,21 @@ newer framework versions (e.g. [`Microsoft.Bcl.AsyncInterfaces`], [`System.Colle
 
 Interoperability with other libraries should be provided in separate packages (e.g. `Funcky.Xunit`, [`Funcky.NewtonsoftJson`])
 
+## 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.
+
 
 [good first issue]: https://github.com/polyadic/funcky/labels/good%20first%20issue
 [mdBook]: https://github.com/rust-lang/mdBook