Create SECURITY.md policy

Author: troglobit

.github/SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+SMCRoute is a small project, as such we have no possibility to support older versions.
+The only supported version is the latest released on GitHub:
+
+<https://github.com/troglobit/smcroute/releases>
+
+## Reporting a Vulnerability
+
+Contact the project's main author and owner to report and discuss vulnerabilities.