docs: add rollup-plugin-css-only for rollup usage (#24)

Co-authored-by: 三咲智子 Kevin Deng <[email protected]>

Author: just-boris

README.md

@@ -27,12 +27,15 @@ export default defineConfig({
 <details>
 <summary>Rollup</summary><br>
 
+Since Rollup does not support CSS out of the box, you need to use a CSS plugin like [`rollup-plugin-css-only`](https://github.com/thgh/rollup-plugin-css-only).
+
 ```ts
 // rollup.config.js
+import css from 'rollup-plugin-css-only'
 import LightningCSS from 'unplugin-lightningcss/rollup'
 
 export default {
-  plugins: [LightningCSS()],
+  plugins: [LightningCSS(), css()],
 }
 ```