docs(security): add Draft GHSA option

- for ease of use, built-in templates, crediting, single source of truth, etc
- as well as consistency with CD: https://github.com/argoproj/argo-cd/blob/a4b50515381bad9d6db316d49d33efae351c6222/SECURITY.md?plain=1#L68

Signed-off-by: Anton Gilgur <agilgur5@gmail.com>

SECURITY.md

@@ -6,9 +6,10 @@ If you find a security related bug in Argo Workflows, we kindly ask you for resp
 disclosure and for giving us appropriate time to react, analyze and develop a
 fix to mitigate the found security vulnerability.
 
-Please report vulnerabilities by e-mail to the following address:
+Please report vulnerabilities by:
 
-* cncf-argo-security@lists.cncf.io
+* Opening a draft GitHub Security Advisory: https://github.com/argoproj/argo-workflows/security/advisories/new
+* Sending an e-mail to the following address: cncf-argo-security@lists.cncf.io
 
 All vulnerabilities and associated information will be treated with full confidentiality.