Validate contribution docs

[nimbinatus]

Documentation of how to contribute, with increasing detail as the project matures.

I think a link to the contribution docs, plus a link to history, probably works here.

[mohan-shash]

CONTRIBUTING.md demonstrates comprehensive contribution documentation with clear evidence of increasing detail as the project has matured.

Evolution Evidence:

• 9+ significant updates over 23 months (Jul 2024 → Jun 2026)
• Growth from basic dev setup to comprehensive workflows covering multiple build variants, testing strategies, and debugging approaches
• Git history: https://github.com/bootc-dev/bootc/commits/main/CONTRIBUTING.md

Current Coverage:

• Development environment setup
• Pre-contribution guidance (file issue before big PRs)
• 5 different development workflows
• Comprehensive testing (unit, container, TMT)
• Advanced features (composefs, sysext, debugging)
• Pull request process with DCO requirements
• Commit message standards

CNCF Requirement Met: "Documentation of how to contribute, with increasing detail as the project matures" ✅
Close it?

Reference: https://github.com/bootc-dev/bootc/blob/main/CONTRIBUTING.md