Clarify contribution licensing (#570)

* Clarify contribution licensing

* less whitespace

Author: alamb

README.md

@@ -133,6 +133,15 @@ of various features from various contributors, but not to provide the
 implementations ourselves, as we simply don't have the resources.
 
 
+## Licensing
+
+All code in this repository is licensed under the [Apache Software License 2.0](LICENSE.txt).
+
+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
+licensed as above, without any additional terms or conditions.
+
+
 [tdop-tutorial]: https://eli.thegreenplace.net/2010/01/02/top-down-operator-precedence-parsing
 [`cargo fmt`]: https://github.com/rust-lang/rustfmt#on-the-stable-toolchain
 [current issues]: https://github.com/sqlparser-rs/sqlparser-rs/issues