docs: add License and Contributing sections to repo top-level README

- moved license files up to repo top-level directory
- minor cleanup of bdk/README.md License and Contribution sections

README.md

@@ -66,3 +66,19 @@ cargo update -p tempfile --precise "3.6.0"
 # rustls 0.21.2 has MSRV 1.60.0+
 cargo update -p rustls:0.21.6 --precise "0.21.1"
 ```
+
+## 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.