Update APPSERVICES.md to include how to remove an appservice

APPSERVICES.md

@@ -40,6 +40,15 @@ appservice can send requests to the homeserver. You don't need to restart
 Conduit, but if it doesn't work, restarting while the appservice is running
 could help.
 
+### Remove an appservice
+
+To remove an appservice go to your admin room and execute
+
+```@conduit:your.server.name: unregister_appservice <name>```
+
+where `<name>` is of course one of the output of `list_appservices`.
+
+
 ## Appservice-specific instructions
 
 ### Tested appservices