Put env vars use can update into separate file that is built from a template.

CloudronManifest.json

@@ -1,6 +1,7 @@
 {
   "title": "Romm",
   "version": "0.1.0",
+  "upstreamVersion":"4.5.0",
   "author":"Kevin Whitaker <kevin@eyecreate.org>",
   "description":"A beautiful, powerful, self-hosted rom manager and player.",
   "healthCheckPath": "/api/heartbeat",
@@ -18,5 +19,6 @@
   },
   "website":"https://romm.app/",
   "icon":"file://logo.png",
-  "manifestVersion": 2
+  "manifestVersion": 2,
+  "postInstallMessage":"Before setting up the first admin user, please check /app/data/env.sh for some API keys for metadata sources and fill in any you want to use. Put any ROMs in the /app/data/library/roms folder. Please see https://docs.romm.app/latest/Getting-Started/Folder-Structure/ for details on folder structure."
 }