Added note for debugging with serve configurations (SharePoint#2181)

Author: thechriskent

docs/spfx/extensions/guidance/migrate-from-usercustomactions-to-spfx-extensions.md

@@ -353,6 +353,8 @@ The UI elements of the custom footer are rendered using React and a custom React
     
     Notice that when executing the page request, you are prompted with a warning message box with the title "Allow debug scripts?", which asks your consent to run code from localhost for security reasons. Of course, if you want to locally debug and test the solution, you have to allow it to "Load debug scripts."
 
+> Alternatively, you can create serve configuration entries in the `config/serve.json` file in your project to automate the creation of the debug query string parameters as outlined in this document: [Debug SharePoint Framework solutions on modern SharePoint pages](https://docs.microsoft.com/en-us/sharepoint/dev/spfx/debug-modern-pages#debug-sharepoint-framework-extensions-on-modern-sharepoint-pages)
+
 ## Package and host the solution
 
 If you are happy with the result, you are now ready to package the solution and host it in a real hosting infrastructure.