Add CONTRIBUTING guidelines (branching, commits, PRs)

[yepzdk]

Document contribution conventions for the repo, aligned with ITK Dev / itk-dev GitHub guidelines.

Scope

• Branch strategy (feature/issue-{number}-{short-description}, never commit to main).
• Conventional Commits format and required Co-authored-by trailer.
• PR workflow and CHANGELOG expectations.
• Coding standards overview.

Acceptance criteria

• CONTRIBUTING.md (or README section) documents branching, commits, and PR workflow.
• Reviewed and merged via PR.

Part of the Base setup milestone.