Add post-install note to mqtt.json (#783)

Author: havardthom

json/mqtt.json

@@ -30,5 +30,10 @@
         "username": null,
         "password": null
     },
-    "notes": []
+    "notes": [
+        {
+            "text": "You can find post-install info here: `https://github.com/community-scripts/ProxmoxVE/discussions/782`",
+            "type": "info"
+        }
+    ]
 }