Add CONTRIBUTING.md
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+By submitting a pull request, you represent that you have the right to license
+your contribution to Apple and the community, and agree by submitting the patch
+that your contributions are licensed under the [Swift
+license](https://swift.org/LICENSE.txt).
+
+---
+
+Before submitting the pull request, please make sure you have tested your
+changes and that they follow the Swift project [guidelines for contributing
+code](https://swift.org/contributing/#contributing-code).
