Add screenshots to manifest

CloudronManifest.json

@@ -16,6 +16,10 @@
   "website": "https://nodebb.org/",
   "contactEmail": "support@cloudron.io",
   "icon": "logo.png",
-  "mediaLinks": [ ],
+  "mediaLinks": [
+    "https://raw.githubusercontent.com/cloudron-io/nodebb-app/e35814fa56fb881d0e28bf71681e63a11bd12740/screenshots/1.png",
+    "https://raw.githubusercontent.com/cloudron-io/nodebb-app/e35814fa56fb881d0e28bf71681e63a11bd12740/screenshots/2.png",
+    "https://raw.githubusercontent.com/cloudron-io/nodebb-app/e35814fa56fb881d0e28bf71681e63a11bd12740/screenshots/3.png"
+  ],
   "memoryLimit": 524288000
 }