Update on SpringBoot : Learn By Example book

An update announcement for the 'SpringBoot: Learn By Example' book, covering two new additions to existing chapters: configuring multiple databases with JPA in Spring Boot (including manual setup of TransactionManagers, EntityManagerFactoryBeans, and DataSourceInitializers), and exposing JPA entities with bi-directional references via REST. A new Chapter 16 on deploying Spring Boot applications has also been added, covering production mode execution, deployment to Heroku via GitHub integration, and running Spring Boot with JPA and Postgres in Docker containers using Docker Compose.