[Docs] Add table of contents for contributor docs (#36861)

* Add table of contents for contributor docs

* add apache license

Author: jrmccluskey

contributor-docs/README.md

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+<!--
+Licensed 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 Documentation
+
+This directory contains documentation for contributors to the Apache Beam project.
+
+## Table of Contents
+
+- [Code Change Guide](code-change-guide.md): A guide for Beam users and developers on how to change, test, and build Beam code, including setting up Java and Python development environments.
+- [Committer Guide](committer-guide.md): Guidelines for Beam committers regarding code review, pull request objectives, merging processes, and post-merge tasks.
+- [Committer Onboarding](committer-onboarding.md): A checklist for new Beam committers to set up their accounts and permissions.
+- [Java Dependency Upgrades](java-dependency-upgrades.md): Instructions for upgrading Java dependencies in Beam, including running linkage checkers and verification tests.
+- [Python Tips](python-tips.md): Tips and instructions for developing the Python SDK, including environment setup, running tests, and handling dependencies.
+- [RC Testing Guide](rc-testing-guide.md): A guide for testing Beam Release Candidates (RCs) against downstream projects for Python, Java, and Go SDKs.
+- [Release Guide](release-guide.md): A comprehensive guide for the Release Manager on how to perform a Beam release, from preparation to promotion.
+- [Updating Supported Python Versions](updating-supported-python-versions.md): Instructions for adding support for new Python versions or removing support for end-of-life versions in Apache Beam.
+
+## Discussion Documents
+
+The [discussion-docs](discussion-docs/) directory contains discussion documents and proposals that have been mailed to the [email protected] mailing list, organized by year.
+
+## Confluence Wiki
+
+The [Confluence wiki](https://cwiki.apache.org/confluence/display/BEAM) for Apache Beam contains additional documentation and information for contributors. Some information may be out of date, but is still useful for reference.