Replace main with demo and explain control flow

Author: prmr

README.adoc

@@ -141,6 +141,8 @@ include::complete/src/main/java/com/example/accessingdatajpa/AccessingDataJpaApp
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+`Application` includes a `demo()` method that puts the `CustomerRepository` through a few tests. First, it fetches the `CustomerRepository` from the Spring application context. Then it saves a handful of `Customer` objects, demonstrating the `save()` method and setting up some data to work with. Next, it calls `findAll()` to fetch all `Customer` objects from the database. Then it calls `findOne()` to fetch a single `Customer` by its ID. Finally, it calls `findByLastName()` to find all customers whose last name is "Bauer". The `demo()` method returns a `CommandLineRunner` bean that automatically runs the code when the application launches.
+
 The `AccessingDataJpaApplication` class includes a `main()` method that puts the
 `CustomerRepository` through a few tests. First, it fetches the `CustomerRepository` from
 the Spring application context. Then it saves a handful of `Customer` objects,