Update readme.md to link to Spring Boot docs

Fixes #1616

readme.md

@@ -47,7 +47,7 @@ In its default configuration, Petclinic uses an in-memory database (H2) which
 gets populated at startup with data. The h2 console is exposed at `http://localhost:8080/h2-console`,
 and it is possible to inspect the content of the database using the `jdbc:h2:mem:<uuid>` URL. The UUID is printed at startup to the console.
 
-A similar setup is provided for MySQL and PostgreSQL if a persistent database configuration is needed. Note that whenever the database type changes, the app needs to run with a different profile: `spring.profiles.active=mysql` for MySQL or `spring.profiles.active=postgres` for PostgreSQL.
+A similar setup is provided for MySQL and PostgreSQL if a persistent database configuration is needed. Note that whenever the database type changes, the app needs to run with a different profile: `spring.profiles.active=mysql` for MySQL or `spring.profiles.active=postgres` for PostgreSQL. See the [Spring Boot documentation](https://docs.spring.io/spring-boot/how-to/properties-and-configuration.html#howto.properties-and-configuration.set-active-spring-profiles) for more detail on how to set the active profile.
 
 You can start MySQL or PostgreSQL locally with whatever installer works for your OS or use docker: