Make licensing more explicit

Cargo.toml

@@ -5,7 +5,7 @@ documentation = "https://docs.rs/num-bigint"
 homepage = "https://github.com/rust-num/num-bigint"
 keywords = ["mathematics", "numerics", "bignum"]
 categories = [ "algorithms", "data-structures", "science" ]
-license = "MIT/Apache-2.0"
+license = "MIT OR Apache-2.0"
 name = "num-bigint"
 repository = "https://github.com/rust-num/num-bigint"
 version = "0.3.1"

README.md

@@ -64,3 +64,18 @@ table offers a brief comparison to a few alternatives.
 [`rust-gmp`]: https://crates.io/crates/rust-gmp
 [`ramp`]: https://crates.io/crates/ramp
 [`apint`]: https://crates.io/crates/apint
+
+## 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.