chore: add SECURITY.md (#720)

Co-authored-by: google-cloud-policy-bot[bot] <80869356+google-cloud-policy-bot[bot]@users.noreply.github.com>
Co-authored-by: gcf-merge-on-green[bot] <60162190+gcf-merge-on-green[bot]@users.noreply.github.com>
Co-authored-by: Justin Beckwith <[email protected]>

Author: 3 people

SECURITY.md

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+# Security Policy
+
+To report a security issue, please use [g.co/vulnz](https://g.co/vulnz).
+
+The Google Security Team will respond within 5 working days of your report on g.co/vulnz.
+
+We use g.co/vulnz for our intake, and do coordination and disclosure here using GitHub Security Advisory to privately discuss and fix the issue.