Update SECURITY.md with advisory creation instructions

Added instructions for creating a new security advisory.

Author: sbrunner

SECURITY.md

@@ -12,3 +12,5 @@
 | 3.32    | Unsupported     |
 | 3.33    | 31/08/2026      |
 | 4.0     | Best effort     |
+
+Never create an issue to notify us about a vulnerability, create a [new security advisory](https://github.com/mapfish/mapfish-print/security/advisories/new).