[homeconnect] Update README.md (openhab#16943)

* Update README.md

Added note for possible occurrence of 403 error during application registration in homeconnect developer portal and recommended a solution to resolve the error.

Author: Linsenschisdre

bundles/org.openhab.binding.homeconnect/README.md

@@ -110,6 +110,9 @@ After the bridge has been added and authorized, devices are discovered automatic
     for example: `http://192.168.178.34:8080/homeconnect` or `https://myhome.domain.com/homeconnect`
     - _One Time Token Mode_: keep unchecked
     - _Proof Key for Code Exchange_: keep unchecked
+      
+       > **NOTE**: You might get an error (`403 - Forbidden`) in case you entered a bare IP of your local openHAB instance. In that case, use your instance's hostname instead (e.g. for openhabian `http://openhabian:8080/homeconnect`) and try again.
+    
 1. After your application has been created, you should see the _Client ID_ and _Client Secret_ of the application. Please save these for later.  
 
 ![Screenshot Home Connect application page](doc/home_connect_application.png "Screenshot Home Connect application page")