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

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm