Expand CONTRIBUTING.md with dev environment setup, commit message standards, and PR guidelines #199

Summary

The current CONTRIBUTING.md is minimal and leaves new contributors without
practical guidance to get started confidently. While it mentions running
make verify and make test before submitting a PR, it lacks the foundational
details that reduce friction for first-time contributors.

What's Currently Missing

1. Development Environment Setup

• No prerequisites listed (Go version, kubectl, required tools)
• No steps to clone, build, or run the project locally
• No mention of IDE recommendations or editor config

2. Commit Message Standards

• The doc says to write "good commit messages" but provides no format or examples
• No mention of conventional commits, scope conventions, or subject line length limits
• Example of expected format would help (e.g., fix(scheduler): correct replica count overflow)

3. PR Checklist and Template

• No PR checklist for contributors to self-review before submitting
• No guidance on PR size (when to split a large PR)
• No mention of linking related issues in the PR description

4. Review Timeline Expectations

• Contributors have no idea how long to wait before following up
• No SLA or approximate timeline mentioned for first review or approval

5. Testing Requirements

• make test is mentioned but there's no guidance on what tests to write
• No direction on unit vs. integration tests, or coverage expectations

Suggested Improvement

Expand CONTRIBUTING.md (or add a linked docs/contributing/ subfolder) to include:

• Prerequisites and local setup steps
• Commit message format with examples
• PR checklist template
• Review response timeline guidance
• Testing expectations and examples

Why This Matters

A clear, practical contributing guide directly lowers the barrier for new
contributors. Many open-source contributors abandon their first PR attempt due
to uncertainty — not lack of skill. Improving this document aligns with
Karmada's stated goal of being "a community project driven by its community."

References

• Current file: CONTRIBUTING.md
• Related: community-membership.md (references contribution history for
  membership tiers — clearer contribution steps help candidates meet those
  requirements too)