Add SECURITY.md referring to the appropriate doc sections. closes https://github.com/opnsense/core/issues/6365

SECURITY.md

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+# Security Policy
+
+More details about our security policy can be found in our documentation
+here https://docs.opnsense.org/security.html
+
+## Supported Versions
+
+OPNsense is a security platform which is released on regular intervals. In
+order to find the latest still supported version, please visit https://docs.opnsense.org/releases.html 
+
+For both the community and business edition only the last published major
+branches are supported in terms of security updates. Major versions will be
+released every 6 months using the following scheme:
+
+XX.1	- January release Community
+XX.7	- July release Community
+XX.4	- April release Business
+XX.10	- Oktober release Business
+
+Where XX equals the year (e.g. 23 for 2023)
+
+## Reporting a Vulnerability
+
+Please check https://docs.opnsense.org/security.html#reporting-an-incident