chore: add SECURITY.md (#47)
chore: add SECURITY.md (#47)

Co-authored-by: google-cloud-policy-bot[bot] <92a6cf9f50641a93109ccdcf7086298233589fca@users.noreply.github.com>
googleapis/google-auth-library-swift,googleapis/google-auth-library-swift,googleapis/google-auth-library-swift