Update README & sample cfg with livekit config

In line with the livekit updates to GroupCallView etc, modify README.md
and the sample config file to contain a working configuration sample for
LiveKit.

config/config.sample.json

@@ -5,5 +5,8 @@
       "server_name": "call.ems.host"
     }
   },
+  "livekit": {
+      "livekit_service_url": "http://localhost:7881"
+    },
   "eula": "https://static.element.io/legal/online-EULA.pdf"
 }