Update Security policy

SECURITY.md

@@ -6,4 +6,6 @@ Only the latest stable version is supported.
 
 ## Reporting a Vulnerability
 
-Send an email to `security AT miniflux DOT net` with all the steps to reproduce the problem.
+Preferably, [report the vulnerability privately using GitHub](https://github.com/miniflux/v2/security/advisories/new) ([documentation](https://docs.github.com/en/code-security/security-advisories/guidance-on-reporting-and-writing/privately-reporting-a-security-vulnerability)).
+
+If you do not want to use GitHub, send an email to `security AT miniflux DOT net` with all the steps to reproduce the problem.