docs: add more info about IIFE standalone demo

Author: ghiscoding

packages/excel-builder-vanilla/README.md

@@ -28,6 +28,9 @@ const { createWorksheet } = require('excel-builder-vanilla');
 
 // IIFE - CDN
 <script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/excel-builder.iife.js"></script>
+<script>
+  const worksheet = ExcelBuilder.createWorksheet();
+</script>
 ```
 
 ## Changelog