Update README with demo image and dev instructions

Added an image to the README and clarified development commands.

Signed-off-by: Vitor Mattos <vitor@php.rio>

Author: vitormattos

README.md

@@ -9,6 +9,8 @@ A Vue 3 component for rendering PDFs with draggable and resizable element overla
 
 **[Demo](https://libresign.github.io/pdf-elements/)** · [Examples](examples/)
 
+<img width="754" height="607" alt="image" src="https://github.com/user-attachments/assets/65009896-21ab-4ec5-9548-2707f8d16cdf" />
+
 ## Development
 
 - `npm run dev` - Run the demo with Vite