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.
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644
index 0000000..a0c1644
--- /dev/null
+++ b/CONTRIBUTING.md
@@ -0,0 +1,14 @@
+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).
+
+---
+
+Changes to this repository follow special considerations as described on
+Swift.org under "[LLVM and Swift](https://swift.org/contributing/#llvm-and-swift)".
+Please make sure your change is appropriate for this repository.
+
+Before submitting a 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).
