[BEAM-13780] Add CONTRIBUTING.md pointing to main guide (apache#16666)

Co-authored-by: Danny McCormick <[email protected]>

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 Beam
+
+*Before opening a pull request*, review the
+[Beam contribution guide](https://beam.apache.org/contribute/).
+It lists steps that are required before creating a PR. In particular, consider
+
+- Have you searched for existing, related JIRAs and pull requests?
+- Have you shared your intent by creating an issue in JIRA and self-assigned?
+- If the change is large, have you discussed it on the dev@ mailing list?
+- Is the change being proposed clearly explained and motivated?