doc: add note about sync nodejs-private branches

PR-URL: #57404
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>

Author: RafaelGSS

doc/contributing/security-release-process.md

@@ -81,6 +81,7 @@ The current security stewards are documented in the main Node.js
 * [ ] 6\. **Get release volunteers:**
   * Get volunteers for the upcoming security release on the affected release
     lines.
+  * Make sure to sync nodejs-private (vN.x) branches with nodejs/node.
 
 * [ ] 7\. **Preparing Pre and Post Release Blog Posts:**
   * [ ] Create a pre-release blog post using `git node security --pre-release`.