chore: add SECURITY.md (GoogleCloudPlatform#5647)

google-cloud-policy-bot[bot] · web-flow · commit 2bae6ae88689 · 2021-07-21T08:57:31.000-07:00
Co-authored-by: google-cloud-policy-bot[bot] &lt;80869356+google-cloud-policy-bot[bot]@users.noreply.github.com&gt;
diff --git a/SECURITY.md b/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.
