Add more configuration information

README.md

@@ -43,6 +43,15 @@ dbp_relay_checkin:
   # campus_qr_token: '%env(CAMPUS_QR_TOKEN)%'
 ```
 
+You also need to set an environment variable `MESSENGER_TRANSPORT_DSN` in your `.env` file or by any other means.
+[Redis](https://redis.io/) is also the best way for this.
+
+Example:
+
+```dotenv
+MESSENGER_TRANSPORT_DSN=redis://redis:6379/local-messages/symfony/consumer?auto_setup=true&serializer=1&stream_max_entries=0&dbindex=0
+```
+
 For more info on bundle configuration see <https://symfony.com/doc/current/bundles/configuration.html>.
 
 ## Roles