docs(readme): document allowed-packages

Author: javier-godoy

README.md

@@ -114,3 +114,14 @@ For a more comprehensive example, see com.flowingcode.vaadin.addons.carousel.Dem
     	Button first = new Button("|<");
     	first.addClickListener(e->cf.movePos(0));
 
+## 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. 
+
+To do so, just add `com.flowingcode` to the `vaadin.allowed-packages` property (Vaadin 14-23: `vaadin.whitelisted-packages`) in `src/main/resources/application.properties`, like:
+
+```
+vaadin.allowed-packages = com.vaadin,org.vaadin,dev.hilla,com.flowingcode
+```
+
+More information on Spring scanning configuration [here](https://vaadin.com/docs/latest/integrations/spring/configuration/#configure-the-scanning-of-packages).