docs(readme): add special configuration when using Spring

Author: javier-godoy

README.md

@@ -88,3 +88,7 @@ Then, follow these steps for creating a contribution:
 This add-on is distributed under Apache License 2.0. For license terms, see LICENSE.txt.
 
 TwinColGrid Add-on is written by Flowing Code S.A.
+
+## Special configuration when using Spring
+
+By default, Vaadin Flow only includes ```com/vaadin/flow/component``` to be always scanned for UI components and views. For this reason, the add-on might need to be allowed in order to display correctly.