Create SECURITY.md

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+Only the latest version is supported. When in doubt visit mailu.io or setup.mailu.io. These pages always show the current version.
+Master is the development branch. Security issues can be reported for this branch as well, but it is not advised to use master for production purposes.
+
+
+| Version         | Supported          |
+| -------         | ------------------ |
+| master          | :white_check_mark: |
+| current version | :white_check_mark: |
+| < current version | :x:                |
+
+## Reporting a Vulnerability
+
+Severe security issues can be reported via security @ mailu.io .
+For Minor security issues create an issue in the Mailu/Mailu repository.