Adding Microsoft SECURITY.MD

[microsoft-github-policy-service[bot]]

Please accept this contribution adding the standard Microsoft SECURITY.MD :lock: file to help the community understand the security policy and how to safely report security issues. GitHub uses the presence of this file to light-up security reminders and a link to the file. This pull request commits the latest official SECURITY.MD file from https://github.com/microsoft/repo-templates/blob/main/shared/SECURITY.md.

Microsoft teams can learn more about this effort and share feedback within the open source guidance available internally.