Add documentationUrl

CloudronManifest.json

@@ -27,6 +27,8 @@
     "storage",
     "sync"
   ],
+  "minBoxVersion": "1.8.1",
+  "documentationUrl": "https://cloudron.io/documentation/apps/syncthing/",
   "mediaLinks": [
     "https://git.cloudron.io/dswd/syncthing-app/raw/master/screenshots/1.jpg"
   ],