Change docs to include the usage of our own TURN server instead of the TURN server of the Open Relay Project

rtc_config_example.json

@@ -3,6 +3,11 @@
   "iceServers": [
     {
       "urls": "stun:stun.l.google.com:19302"
+    },
+    {
+      "urls": "turn:example.com:3478",
+      "username": "username",
+      "credential": "password"
     }
   ]
 }