Create SECURITY.md (#144)

Author: plumpy

SECURITY.md

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+# Reporting Security Issues
+
+To report a security issue, please use http://g.co/vulnz. We use
+http://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.
+
+[GitHub Security Advisory]: https://github.com/GoogleContainerTools/skaffold/security/advisories