[WIP] Add ROADMAP.md file for version planning

[Copilot]

• Create ROADMAP.md at the repository root with the suggested structure from the issue

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add ROADMAP.md</issue_title>
<issue_description>## Summary

Add a ROADMAP.md file to the repository root listing what is planned for v1.0.0 and beyond.

Suggested structure

# Roadmap

## v1.0.0 (stable API)
- [ ] Stable public API surface documented in `docs/stability.md`
- [ ] Coverage threshold enforced at 90%
- [ ] Strict Mypy enabled
- [ ] Release notes automated via git-cliff

## Future
- Autocorrelation / ACF (#19)
- skfolio integration (#96)
- Mutation testing

Why

Visitors evaluating whether to adopt a library look for signals of active maintenance and a clear direction. A ROADMAP.md converts passive evaluators into active watchers and future contributors.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add ROADMAP.md #419

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.