add ASF license header

guides/ml-in-Wayang.md

@@ -1,3 +1,21 @@
+<!--
+
+  Licensed to the Apache Software Foundation (ASF) under one or more
+  contributor license agreements.  See the NOTICE file distributed with
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+  The ASF licenses this file to You under the Apache License, Version 2.0
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+
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+
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+-->
 # Using Machine Learning for query optimization in Apache Wayang (incubating)
 Apache Wayang (incubating) can be customized with concrete
 implementations of the `EstimatableCost` interface in order to optimize