Add reference to mobx-react-observer (#4542)

As suggested by @mweststrate : #4537 (comment)

Author: christianalfoni

docs/react-integration.md

@@ -8,7 +8,7 @@ hide_title: true
 
 # React integration
 
-Usage:
+This documentation outlines how to manually apply observation to React components. However, by using the [mobx-react-observer](https://github.com/christianalfoni/mobx-react-observer) Babel/SWC plugin, you can automatically handle observation without manual intervention. Still, understanding how MobX observation integrates with React components remains valuable, even when leveraging automated solutions.
 
 ```javascript
 import { observer } from "mobx-react-lite" // Or "mobx-react".