Add installation guide to Readme

Author: AB-xdev

README.md

@@ -13,6 +13,9 @@ A Vaadin Template Repo
 ## Installation
 [Installation guide for the latest release](https://github.com/xdev-software/template-placeholder/releases/latest#Installation)
 
+### Additional notes
+* [Spring-Boot] You may have to include ``software/xdev`` inside [``vaadin.allowed-packages``](https://vaadin.com/docs/latest/integrations/spring/configuration#configure-the-scanning-of-packages)
+
 ## Run the Demo
 * Checkout the repo
 * Run ``mvn install && mvn -f template-placeholder-demo spring-boot:run``