add license and contribution notices to README.md

README.md

@@ -21,4 +21,21 @@ assert_eq!(iter.next(), Some(6));
 assert_eq!(iter.next(), Some(24));
 assert_eq!(iter.next(), Some(120));
 assert_eq!(iter.next(), None);
-```
+```
+
+## 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.