Add license info to README.md

Author: nvzqz

README.md

@@ -18,7 +18,21 @@ This is a cross-platform framework that serves to bring the majority of Hexe's f
 
 Because Rust and Swift don't map one-to-one, this API may diverge slightly from the Rust API.
 
+## License
+
+Hexe.swift is available under the same license(s) as [Hexe], either of
+
+ - [Apache License, Version 2.0][license-apache]
+
+ - [MIT License][license-mit]
+
+at your option.
+
+[Hexe]: https://github.com/hexe-rs/Hexe
 [Rust]: https://www.rust-lang.org/
 
 [travis]:       https://travis-ci.org/hexe-rs/Hexe.swift
 [travis-badge]: https://travis-ci.org/hexe-rs/Hexe.swift.svg?branch=master
+
+[license-apache]: https://github.com/hexe-rs/Hexe.swift/blob/master/LICENSE-APACHE
+[license-mit]:    https://github.com/hexe-rs/Hexe.swift/blob/master/LICENSE-MIT