Create SECURITY.md (nginx#264)

Author: miledxz

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+We advise users to run the most recent release of the NGINX Kubernetes Gateway. A table with the supported versions can be found in the [README](README.md#technical-specifications).
+
+## Reporting a Vulnerability
+
+The F5 Security Incident Response Team (F5 SIRT) has an email alias that makes it easy to report potential security vulnerabilities.
+
+Please report any potential or current instances of security vulnerabilities with any F5 product to the F5 Security Incident Response Team at F5SIRT@f5.com
+
+For more information visit https://www.f5.com/services/support/report-a-vulnerability