Add license to README

Author: poojanilangekar

extensions/federation/README.md

@@ -1,3 +1,21 @@
+<!--
+  Licensed to the Apache Software Foundation (ASF) under one
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+  "License"); you may not use this file except in compliance
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+ 
+   http://www.apache.org/licenses/LICENSE-2.0
+ 
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+-->
 ### Using the `HadoopFederatedCatalogFactory`
 
 This `HadoopFederatedCatalogFactory` module is an independent compilation unit and will be built into the Polaris binary only when the following flag is set in the gradle.properties file: