docs: update configuration guide for port changes affecting PDF generation

Add note explaining that when changing the default port in Docker Compose setup,
the HostName in appsettings.json must also be updated to include the new port
number for PDF generation to work correctly.

Author: PeRobin

README.md

@@ -181,6 +181,15 @@ Sometimes the local environment have another service running on port 80, so you
 ALTINN3LOCAL_PORT=80
 ```
 
+> [!NOTE]
+> If you change the port, you must also update the `HostName` in `appsettings.json` in your form application for PDF generation to work correctly. For example:
+> 
+> ```json
+>"GeneralSettings": {
+>  "HostName": "local.altinn.cloud:<PORTNUMBER>",  
+> }
+> ```
+
 If you want to see the storage files on disk (instead of reading them through the browser), change this to a local
 path on your computer (ensure that it exists)