docs: add SECURITY.md

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Here are the versions of Quickemu currently being supported with security updates.
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 4.9.x   | :white_check_mark: |
+| < 4.8   | :x:                |
+
+## Reporting a Vulnerability
+
+If you discover a vulnerability in Quickemu then [file an issue](https://github.com/quickemu-project/quickemu/issues/new) prefixed `security:`
+
+Quickemu is a spare-time hobby project.
+We do not have SLAs for responding to security issues.
+It is a best-efforts basis.