relicense under dual MIT/Apache scheme

see
- https://rust-lang.github.io/api-guidelines/necessities.html

README.md

@@ -41,4 +41,16 @@ fn read_lume_to_string() -> String {
 
 ## License
 
-[MIT](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.