Mention that a custom starter must bring the core starter

Closes gh-11455

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

@@ -6505,6 +6505,11 @@ a proper set of _default_ dependencies may be hard if the number of optional dep
 is high as you should avoid bringing unnecessary dependencies for a typical usage of the
 library.
 
+NOTE: Either way, your starter must reference the core Spring Boot starter
+(`spring-boot-starter`) directly or indirectly (i.e. no need to add it if your starter
+relies on another starter). If a project is created with only your custom starter, Spring
+Boot's core features will be honoured by the presence of the core starter.
+
 
 
 [[boot-features-whats-next]]