Polish contributing.adoc

Issue #938

Signed-off-by: etrandafir93 <[email protected]>

Author: etrandafir93

CONTRIBUTING.adoc

@@ -81,6 +81,8 @@ The wiki pages https://github.com/spring-projects/spring-framework/wiki/Code-Sty
 
 To format the code as well as check the style, run `./gradlew format check`.
 
+Lastly, run `./gradlew checkstyleMain checkstyleTest` before submitting a pull request, for additional style checks.
+
 [[submit-a-pull-request]]
 === Submit a Pull Request