Create SECURITY.md

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+As of 2021/08 -
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 'dev'   | :x: GitHub 'master' branch, use at own risk! |
+| 3.3.x   | :white_check_mark: |
+| 3.2.x   | Security/critical fixes only |
+| < 3.2.x | :x:                |
+
+Releases are published at :
+
+ * https://github.com/postfixadmin/postfixadmin/releases
+ * ocassionally at https://sourceforge.net/projects/postfixadmin/ - sometimes with RPM/DEB packages.
+
+## Reporting a Vulnerability
+
+Either message GingerDog or cboltz on the PostfixAdmin libera chat - IRC channel, or email. Email addresses can be found in the 'git' changelog.
+
+