New security policy: reporting vulnerabilities in the Swift project

The Swift project now uses GitHub's private vulnerability reporting for repositories in the swiftlang organization. The security policy documents how to report a vulnerability and what to expect after you do. If you believe you've found a security vulnerability in a Swift.org project, please report it privately: Go to the Security tab of the repository. Select Report a vulnerability. Full details are in the security policy. Thank you to everyone in the community who helps keep Swift secure.