Add community contribution guidelines

CONTRIBUTING.md

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+# Contributing to proxy.py
+
+This document describes how contributors can participate and iterate quickly while maintaining the `proxy.py` project standards and guidelines.
+
+## Basic Guidelines
+
+* Your pull request should NOT introduce any external dependency.
+* It is OK to use external dependencies within plugins.
+
+## Environment Setup
+
+Contributors must start `proxy.py` from source to verify and develop new features / fixes.  See [Run proxy.py from command line using repo source](#from-command-line-using-repo-source) for usage instructions.
+
+[![WARNING](https://img.shields.io/static/v1?label=MacOS&message=warning&color=red)](https://github.com/abhinavsingh/proxy.py/issues/642#issuecomment-960819271) On `macOS` you must install `Python` using `pyenv`, as `Python` installed via `homebrew` tends to be problematic.  See linked thread for more details.
+
+### Setup Git Hooks
+
+You SHOULD NOT avoid these steps.  Git hooks will help catch test or linting errors locally without pushing to upstream.  This will save you a lot of time and allow you to contribute and iterate faster.
+
+Pre-commit hook ensures tests are passing.
+
+1. `cd /path/to/proxy.py`
+2. `ln -s $(PWD)/git-pre-commit .git/hooks/pre-commit`
+
+Pre-push hook ensures lint and tests are passing.
+
+1. `cd /path/to/proxy.py`
+2. `ln -s $(PWD)/git-pre-push .git/hooks/pre-push`
+
+### Sending a Pull Request
+
+All pull requests are tested using GitHub actions.
+
+See [GitHub workflow](https://github.com/abhinavsingh/proxy.py/tree/develop/.github/workflows) for list of workflows.
+
+## Communication
+
+During the process of PR review, sometimes, you may get asked to update certain project configs.  Example, a change in code introduced via your PR will result in a redundant lint guard.  So we must make corresponding changes to ensure project health.
+
+It's highly recommended that you participate in maintaining a high code-quality standard.  For any reason, if you are unable to address the requested changes, please communicate the same to the reviewer.
+
+Thank you!!!