add license terms to README

Author: newpavlov

README.md

@@ -52,3 +52,18 @@ Release notes are available in [RELEASES.md](RELEASES.md).
 ## Compatibility
 
 The `num-traits` crate is tested for rustc 1.8 and greater.
+
+## License
+
+Licensed under either of
+
+ * [Apache License, Version 2.0](http://www.apache.org/licenses/LICENSE-2.0)
+ * [MIT license](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.