Add legal notice for pull requests and reference to contribution guidelines.

GitHub will look for this file when a new pull request is opened and offer
it to the user.

Author: jrose-apple

Diff for: CONTRIBUTING.md

@@ -0,0 +1,10 @@
+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).