Add license header

Author: gabotechs

datafusion/physical-plan/src/buffer.rs

@@ -1,3 +1,23 @@
+// 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
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//! [`BufferExec`] decouples production and consumption on messages by buffering the input in the
+//! background up to a certain capacity.
+
 use crate::execution_plan::CardinalityEffect;
 use crate::filter_pushdown::{
     ChildPushdownResult, FilterDescription, FilterPushdownPhase,