doc: add a security policy

SECURITY.md

+# Security Policy
+
+## Supported Versions
+
+Security fixes and updates are only applied to the latest released version. So always try to be up to date.
+
+## Reporting a Vulnerability
+
+In order to minimize risks of attack while investigating and fixing the issue, any vulnerability shall be reported by 
+opening a [**confidential** issue on gitlab.com](https://gitlab.com/to-be-continuous/sonar/-/issues/new).
+
+Follow-up and fixing will be made on a _best effort_ basis.
+
+If you have doubts about a potential vulnerability, please reach out one of the maintainers on Discord.