Add router event note (#7736)

* Add router event note

* Add line break

* Tweak wording

packages/next/README.md

@@ -815,6 +815,9 @@ Router.events.on('routeChangeError', (err, url) => {
 })
 ```
 
+> **Note**: Using router events in `getInitialProps` is discouraged as it may result in unexpected behavior.<br/>
+> Router events should be registered when a component mounts (`useEffect` or `componentDidMount`/`componentWillUnmount`) or imperatively when an event happens.
+
 ##### Shallow Routing
 
 <details>