add license header

Author: novatechflow

guides/spark-datasets.md

@@ -1,3 +1,22 @@
+<!--
+
+  Licensed to the Apache Software Foundation (ASF) under one or more
+  contributor license agreements.  See the NOTICE file distributed with
+  this work for additional information regarding copyright ownership.
+  The ASF licenses this file to You under the Apache License, Version 2.0
+  (the "License"); you may not use this file except in compliance with
+  the License.  You may obtain a copy of the License at
+
+      http://www.apache.org/licenses/LICENSE-2.0
+
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+-->
+
 ---
 title: Spark Dataset pipelines
 description: How to build Wayang jobs that stay on Spark Datasets/DataFrames from source to sink.