Explain how to build the reference documentation

README.adoc

@@ -112,6 +112,24 @@ https://support.springsource.com/spring_committer_signup[Contributor's Agreement
 before your first change however trivial. (Or if you filed such an agreement already for
 another project just mention that in your pull request.)_
 
+=== Building reference documentation
+
+The reference documentation requires the documentation of the maven plugin to be
+available so you need to build that first since it's not generated by default.
+
+[indent=0]
+----
+	$ mvn clean install -pl spring-boot-tools/spring-boot-maven-plugin -Pdefault,full
+----
+
+Once this is done, you can build the reference documentation with the command below:
+
+[indent=0]
+----
+	$ mvn clean install -pl spring-boot-docs -Pdefault,full
+----
+
+TIP: The generated documentation is available from `spring-boot-docs/target/contents/reference`
 
 
 == Modules