Add missing license header

Author: leekeiabstraction

docs/rust-client.md

@@ -1,3 +1,21 @@
+<!--
+ Licensed to the Apache Software Foundation (ASF) under one
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+ regarding copyright ownership.  The ASF licenses this file
+ to you under the Apache License, Version 2.0 (the
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+ with the License.  You may obtain a copy of the License at
+
+      http://www.apache.org/licenses/LICENSE-2.0
+
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+-->
+
 # Rust Client Guide
 
 This guide covers how to use the Fluss Rust client for reading and writing data to log tables and primary key tables.