Add license terms to the README

Author: cuviper

README.md

@@ -40,11 +40,25 @@ Implementations for `i128` and `u128` are only available with Rust 1.26 and
 later.  The build script automatically detects this, but you can make it
 mandatory by enabling the `i128` crate feature.
 
-
 ## Releases
 
 Release notes are available in [RELEASES.md](RELEASES.md).
 
 ## Compatibility
 
 The `num-integer` 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.