Clarify Maven plugin usage

Author: Dave Syer

spring-boot-docs/src/main/asciidoc/using-spring-boot.adoc

@@ -125,8 +125,9 @@ section if you want to use it:
 	</build>
 ----
 
-NOTE: You only need to add the plugin, there is no need for to configure it unless you
-want to change the settings defined in the parent.
+NOTE: If you use the Spring Boot starter parent pom, you only need to
+add the plugin, there is no need for to configure it unless you want
+to change the settings defined in the parent.