Added missing certfile/keyfile entries to code (home-assistant#4116)

Zorks · frenck · commit c8015334846f · 2017-12-01T20:47:27.000+01:00
If user accidentally removes those by copy/pasting what was listed, it throws a "missing" error.
diff --git a/source/_addons/mosquitto.markdown b/source/_addons/mosquitto.markdown
@@ -23,7 +23,9 @@ Set up [Mosquitto](https://mosquitto.org/) as MQTT broker.
   "customize": {
     "active": false,
     "folder": "mosquitto"
-  }
+  },
+  "certfile": "fullchain.pem",
+  "keyfile": "privkey.pem"
 }
 ```
 
