Add Apache Airflow CODE_OF_CONDUCT.md (apache#9715)

CODE_OF_CONDUCT.md

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+<!--
+ 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.
+-->
+
+# Contributor Covenant Code of Conduct
+
+The Apache Airflow project follows the [Apache Software Foundation code of conduct](https://www.apache.org/foundation/policies/conduct.html).
+
+If you observe behavior that violates those rules please follow the [ASF reporting guidelines](https://www.apache.org/foundation/policies/conduct#reporting-guidelines).