ARROW-4006: Add CODE_OF_CONDUCT.md

Many people are not aware that The ASF has a code of conduct. Having this document in the root directory will increase awareness of the type of professional behavior we expect from members of our community.

Author: Wes McKinney <[email protected]>

Closes apache#3179 from wesm/ARROW-4006 and squashes the following commits:

6e88d8a <Wes McKinney> Add CODE_OF_CONDUCT.md

Author: wesm

CODE_OF_CONDUCT.md

@@ -0,0 +1,24 @@
+<!---
+  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.
+-->
+
+# Code of Conduct
+
+* [Code of Conduct for The Apache Software Foundation][1]
+
+[1]: https://www.apache.org/foundation/policies/conduct.html