Add simple README for the format/ directory

Change-Id: I0be8ff87c6ffd352e9c26e01154dcf2da5743873

format/README.md

@@ -0,0 +1,25 @@
+.. 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.
+
+Arrow Protocol Files
+====================
+
+This folder contains binary protocol definitions for the Arrow columnar format
+and other parts of the project, like the Flight RPC framework.
+
+For documentation about the Arrow format, see the `docs/source/format`
+directory.