add doc url

CloudronManifest.json

@@ -24,5 +24,7 @@
   "memoryLimit": 524288000,
   "changelog": "file://CHANGELOG",
   "tags": [ "forum", "collaboration", "community", "discourse", "bb" ],
-  "postInstallMessage": "file://POSTINSTALL.md"
+  "postInstallMessage": "file://POSTINSTALL.md",
+  "minBoxVersion": "1.8.1",
+  "documentationUrl": "https://cloudron.io/documentation/apps/nodebb/"
 }