Add note explaining gradle ResolutionStrategy

Update reference documentation to include a note explaining that
"blessed" versions a chosen based on the gradle plugin version, and
that builds are repeatable.

Fixes gh-619

Author: Phillip Webb

spring-boot-docs/src/main/asciidoc/build-tool-plugins.adoc

@@ -352,6 +352,11 @@ Simply declare dependencies in the usual way, but leave the version number empty
 	}
 ----
 
+NOTE: The version of the `spring-boot` gradle plugin that you declare determines the
+actual versions of the ``blessed'' dependencies (this ensures that builds are always
+repeatable). You should always set the version of the `spring-boot` gradle plugin to the
+actual Spring Boot version that you wish to use.
+
 
 
 [[build-tool-plugins-gradle-packaging]]

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

@@ -146,8 +146,9 @@ Maven, there is no ``super parent'' to import to share some configuration.
 
 The <<build-tool-plugins.adoc#build-tool-plugins-gradle-plugin, `spring-boot-gradle-plugin`>>
 is also available and provides tasks to create executable jars and run projects from
-source. It also adds a `ResolutionStrategy` that enables you to omit the version number
-for ``blessed'' dependencies:
+source. It also adds a `ResolutionStrategy` that enables you to
+<<build-tool-plugins-gradle-dependencies-without-versions, omit the version number
+for ``blessed'' dependencies>>:
 
 [source,groovy,indent=0,subs="attributes"]
 ----