docs: update readme

Author: atinux

README.md

@@ -4,6 +4,8 @@ This template should help get you started developing with Vue 3 and TypeScript i
 
 It also uses [Nuxt UI v3](https://ui3.nuxt.dev) for Vue.
 
+Online demo: https://codesandbox.io/p/github/atinux/nuxt-ui3-vue
+
 ## Recommended IDE Setup
 
 - [VS Code](https://code.visualstudio.com/) + [Volar](https://marketplace.visualstudio.com/items?itemName=Vue.volar) (and disable Vetur) + [TypeScript Vue Plugin (Volar)](https://marketplace.visualstudio.com/items?itemName=Vue.vscode-typescript-vue-plugin).