Update app.json

Author: kyb3r

app.json

@@ -22,6 +22,10 @@
     "MONGO_URI": {
         "description": "Mongo DB connection URI for selfhosting your data.",
         "required": false
+    },
+    "LOG_URL": {
+        "description": "The url of the logviewer app for viewing selfhosted logs",
+        "required": false
     }
   }
 }