Update post install

POSTINSTALL.md

@@ -1,9 +1,10 @@
-This application integrates with Cloudron authentication.
-However, all Cloudron users share the same Syncthing session.
-Please note that only the username is accepted as login and not the email address.
+Use the following credentials for initial setup:
 
-Syncthing contains an internal user `admin` that is needed by Cloudron but not exposed. 
-Please do not change the password of that account.
+`username`: admin
 
-Please create all sync folders as subfolders of `/app/data` as this is the only writable folder.
+`password`: changeme
+
+**Please change the admin password on first login**
+
+Sync folders must be subfolders of `/app/data`.