Create SECURITY.md (#53)

Author: qiwzhang

SECURITY.md

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+# Security Policy
+
+## Follow envoy security policy
+
+This code is mainly used by [envoy](https://github.com/envoyproxy/envoy). Please follow envoy security [policy](https://github.com/envoyproxy/envoy/security/policy).
+
+
+## Supported Versions
+
+This repo is small, there is not versioned releases. Please always use top of the tree.  If there is any bugs or security fixes, they will be fixed in the master branch.
+
+
+## Reporting a Vulnerability
+
+Please report any vulnerability to envoy according to its security [policy](https://github.com/envoyproxy/envoy/security/policy).