Update CONTRIBUTING.md to point at the correct licence file (rust-lang#1803)

Author: danielsn

CONTRIBUTING.md

@@ -54,5 +54,6 @@ See our [security disclosure instructions](.github/SECURITY.md) for more details
 
 ## Licensing
 
-See the [LICENSE](LICENSE) file for our project's licensing. We will ask you to confirm the licensing of your contribution.
-
+Kani is distributed under the terms of both the MIT license and the Apache License (Version 2.0).
+See [LICENSE-APACHE](LICENSE-APACHE) and [LICENSE-MIT](LICENSE-MIT) for details.
+By submitting a pull request, you confirm that your contribution is made under the terms of the Apache 2.0 and MIT licenses.