doc: explain connection issue right after deployment

Makefile

@@ -118,7 +118,7 @@ dependencies-checks:
 .PHONY: goodbye
 goodbye:
 	@echo "\nCongratulations, you successfully deployed MLFlow on your project! 🎉"
-	@source vars && echo "The web app is available at https://mlflow-dot-$${TF_VAR_project_id}.ew.r.appspot.com"
+	@source vars && echo "The web app is available at https://mlflow-dot-$${TF_VAR_project_id}.ew.r.appspot.com (You may have trouble connecting for a few minutes after the deployment, while IAP gets setup)"
 	@echo "To push your first experiment, take a look at the bottom of the readme for an example."