Refrase license reference

Uses a more standard reference to the license recommended by the rust
documentation (https://rust-lang.github.io/api-guidelines/necessities.html)

Signed-off-by: Rafael Garcia Ruiz <rafael.garcia@collabora.com>

Author: Razaloc

README.md

@@ -202,4 +202,18 @@ your file or, otherwise, expand it. Later on, you may reconstruct it using the
 `bmap-rs copy` command.
 
 ## License
-bmap-rs is licensed under dual Apache-2.0 and MIT licenses.
+
+Licensed under either of
+
+ * Apache License, Version 2.0
+   ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+ * MIT license
+   ([LICENSE-MIT](LICENSE-MIT) or 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.