document "occ social:reset" in readme

Signed-off-by: Jonas Sulzer <jonas@violoncello.ch>

README.md

@@ -17,6 +17,8 @@ Some requirements in this alpha stage are that your Nextcloud:
 - **🕸 Open standards:** We use the established [ActivityPub](https://en.wikipedia.org/wiki/ActivityPub) standard!
 - **🎨 Nice illustrations:** Made by [Katerina Limpitsouni of unDraw](https://undraw.co).
 
+## Reset app
+If you want to reset all Social app data e.g. to change the domain used for Social, you can use `occ social:reset` (For how to use occ commands see documentation: [using the occ command](https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/occ_command.html)).
 
 ## Development setup