Cross-reference "Create a deployable war" how-to

Add a tip for both the Maven and Gradle "Packaging executable jar and
war files" section referring to the how-to.

Fixes gh-2086

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

@@ -154,6 +154,9 @@ need to mark the embedded container dependencies as "`provided`", e.g:
 	</project>
 ----
 
+TIP: See the "`<<howto-create-a-deployable-war-file>>`" section for more details on
+how to create a deployable war file.
+
 Advanced configuration options and examples are available in the
 {spring-boot-maven-plugin-site}/[plugin info page].
 
@@ -359,6 +362,10 @@ named "`providedRuntime`", e.g:
 	}
 ----
 
+TIP: See the "`<<howto-create-a-deployable-war-file>>`" section for more details on
+how to create a deployable war file.
+
+
 
 [[build-tool-plugins-gradle-running-applications]]
 === Running a project in-place