Added contributing file (#5)

CONTRIBUTING.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.
+-->
+
+# Contributing to Fluo Docker
+
+Checkout [How to Contribute][contribute] for general instructions on contributing to Fluo
+projects.
+
+Before making a pull request to this project see if the `docker build` command mentioned in
+the [README](README.md) runs successfully.  If it does not succeed and you are unsure why, then go
+ahead and create the pull request with a comment about the failure.
+
+[contribute]: https://fluo.apache.org/how-to-contribute/