Add checkstyle.xml

Source: https://github.com/checkstyle/checkstyle/blob/0332cbe0ba9f74eebf5955b0f98780a422a3caaa/src/main/resources/sun_checks.xml

pom.xml

@@ -130,6 +130,7 @@
                 <artifactId>maven-checkstyle-plugin</artifactId>
                 <version>3.1.2</version>
                 <configuration>
+                    <configLocation>checkstyle.xml</configLocation>
                     <encoding>UTF-8</encoding>
                     <consoleOutput>true</consoleOutput>
                     <failsOnError>true</failsOnError>