Add CITATION.cff and maintainer release citation guidance (#269).

[rosspeili]

Description

Adds root CITATION.cff, a README Citing section, and maintainer release guidance so Skillware has a formal cite path before Zenodo concept-DOI linking.

Refs #269 (Phase A). Phase B (Zenodo connect + DOI in CFF) follows after merge.

Type of Change

• RFC / meta — templates, labels, CI, or large design doc
• Documentation — docs, README, CONTRIBUTING only

Checklist (all PRs)

• Linked GitHub issue (Refs #269)
• Scope matches the issue — no unrelated refactors
• python -m black --check . and flake8 pass locally (or CI-equivalent subset) — N/A (docs/metadata only)
• pytest skills/ and pytest tests/ pass locally when relevant — N/A
• CHANGELOG.md updated under [Unreleased] when user-visible behavior changes
• examples/README.md updated if this PR adds, renames, or removes a runnable script — N/A
• Ran pytest tests/test_registry_docs.py when skills, examples index, or agent-loops matrix changed — N/A

Related Issues

Refs #269