Create SECURITY.md

Author: OllieMartin

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+MultiChat in general only supports the latest version of the plugin. However, we will offer security updates to version 1.7.5 if required. Please note version 1.7.4 and lower is NOT supported. Additionally, while version 1.8.1 is the recommended version, version 1.8 is NOT supported as it contained security vulnerabilities.
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 1.8.1   | :white_check_mark: |
+| 1.8     | :x:                |
+| 1.7.5   | :white_check_mark: |
+| < 1.7.5 | :x:                |
+
+## Reporting a Vulnerability
+
+In order to report a security vulnerability, please contact me privately using Discord (Revilo#3969) or in some other way. I discourage the use of GitHub Issues tracking for reporting security vulnerabilities as this could then form a 'map' for a malicious user to target servers running outdated versions of the software.