Add spring boot guide in free-programming-books.md (#4570)

free-programming-books.md

@@ -1613,6 +1613,7 @@ Kerridge (PDF) (email address *requested*, not required)
 
 #### Spring Boot
 
+* [Building modern Web Apps with Spring Boot and Vaadin](https://v.vaadin.com/hubfs/Pdfs/Building%20Modern%20Web%20Apps%20with%20Spring%20Boot%20and%20Vaadin.pdf) (PDF)
 * [Spring Boot Reference Guide](https://docs.spring.io/spring-boot/docs/current/reference/html/) - Phillip Webb et al. ([PDF](https://docs.spring.io/spring-boot/docs/current/reference/pdf/spring-boot-reference.pdf))