Update README with 5th lesson regarding react-router client setup

The fifth lesson covers setting up react-router on the client-side
so routing can be isomorphic (react-router now uses the same route
configuration for both server and client).

No new routes were created.

YouTube:
- Title: Client-side routing with react-router
- Link: https://www.youtube.com/watch?v=r9rWABuGPpQ

Author: remarkablemark

README.md

@@ -46,6 +46,13 @@ $ git checkout v4
 $ npm start
 ```
 
+[5. Client-side routing with react-router](https://b.remarkabl.org/2faLHsH):
+
+```sh
+$ git checkout v5
+$ npm start
+```
+
 ## License
 
 [MIT](https://github.com/remarkablemark/universal-react-tutorial/blob/master/LICENSE)