Update checkstyle instructions

CONTRIBUTING.md

@@ -6,9 +6,9 @@ the repository and sending a pull request.
 
 When submitting code, please make every effort to follow existing conventions
 and style in order to keep the code as readable as possible. Please also make
-sure your code compiles by running `mvn clean verify`. Checkstyle failures
-during compilation indicate errors in your style and can be viewed in the
-`checkstyle-result.xml` file.
+sure your code compiles by running `./gradlew clean build`. Checkstyle failures
+during compilation indicate errors in your style and can be viewed in files
+located at `*/build/report/checkstyle` directories.
 
 Before your code can be accepted into the project you must also sign the
 [Individual Contributor License Agreement (CLA)][1].