docs(readme): document allowed-packages

Author: javier-godoy

README.md

@@ -102,3 +102,15 @@ For a more comprehensive example, see `com.flowingcode.vaadin.addons.chipfield.D
         Binder<Planet> binder = new Binder<>();
         binder.bind(chf5,Planet::getConfiguration,Planet::setConfiguration);
         binder.setBean(p);
+
+## 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).