Add reference to Spark scalastyle-config.xml

scala/scalastyle-config.xml

@@ -37,6 +37,10 @@ This file is divided into 3 sections:
  (3) rules that we don't want to enforce.
 -->
 
+<!--
+Reference: Spark scalastyle-config.xml (https://github.com/apache/spark/blob/master/scalastyle-config.xml)
+-->
+
 <scalastyle>
   <name>Scalastyle standard configuration</name>