[CDAP-20182] Create SECURITY.md

Author: dli357

SECURITY.md

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