[Enhancement] Add comprehensive CONTRIBUTING.md guide to improve contributor onboarding

[ayoajuwon1]

Problem Statement

The OpenAI Agents SDK repository currently lacks a CONTRIBUTING.md file, which creates friction for potential contributors. As evidenced by the "Want to contribute?" banner linking to a generic open-source guide, there's no project-specific guidance on:

• How to set up the development environment
• Contribution workflow (branching, commits, PRs)
• Code style and testing requirements
• Issue triage and labeling conventions
• Release process and versioning
• Community expectations and code of conduct

With 18.1k stars, 201 contributors, and 155 open issues, standardizing the contribution process would:

1. Reduce maintainer burden by setting clear expectations
2. Lower the barrier to entry for new contributors
3. Improve PR quality through upfront guidance
4. Scale community contributions as the project grows

Proposed Solution

Add a comprehensive CONTRIBUTING.md file that covers:

1. Getting Started

• Prerequisites (Python 3.9+, uv installation)
• Fork and clone workflow
• Development environment setup (make sync)
• Running tests locally (make tests, make mypy, make lint)

2. Development Workflow

• Branch naming conventions (e.g., feature/, fix/, docs/)
• Commit message format (conventional commits)
• How to run the full test suite
• When to add/update documentation

3. Submitting Changes

• PR template expectations
• Required checks (tests, linting, type checking)
• Review process and timeline expectations
• How to respond to feedback

4. Types of Contributions

• Bug fixes with reproduction steps
• Feature requests with use case justification
• Documentation improvements
• Example additions
• Test coverage improvements

5. Code Standards

• Reference to existing tooling (ruff, mypy)
• Type annotation requirements
• Documentation standards (docstrings)
• Test coverage expectations

6. Community Guidelines

• Issue triage labels and their meanings
• How to report security vulnerabilities
• Code of conduct reference
• Communication channels (issues, discussions, Discord if applicable)

Success Metrics

• Reduced "how do I contribute?" questions in issues
• Faster PR review cycles due to better initial quality
• Increased percentage of PRs passing CI on first attempt
• More first-time contributors successfully landing PRs

Additional Context

Many successful OSS projects have comprehensive contributing guides:

• FastAPI Contributing
• Django Contributing
• React Contributing

Given the SDK's rapid growth and the complexity of the agent patterns it enables, clear contribution guidelines would significantly improve the contributor experience and project velocity.

Implementation Notes

I'm happy to draft an initial CONTRIBUTING.md based on the existing Makefile targets and observed PR patterns. This could be refined through maintainer feedback and community input.