Add proposed project roadmap and link it from the README

[Copilot]

This adds a proposed ROADMAP.md for version-checker based on the current milestone themes, open issues, labels, and active pull requests. The roadmap focuses on near-term stability work, architecture and OCI support, and test coverage, while separating mid-term and longer-term items.

• Roadmap document

  • Added ROADMAP.md to capture a scan-friendly plan across:
    • near-term (v1 / H1-Q3) priorities
    • mid-term follow-on work
    • longer-term / future-development items
  • Anchored the plan to the existing project signals already in GitHub:
    • bug backlog
    • architecture-aware image support
    • OCI support
    • test coverage work

• Near-term prioritization

  • Called out open bug and accuracy work as the first tranche of effort
  • Included open PRs with passing checks as near-term candidates rather than treating all in-flight work equally
  • Kept larger or not-yet-green work out of the immediate merge path

• Documentation integration

  • Added a README link so the roadmap is discoverable from the main documentation entry points

Example:

## Near term (H1 / Q3)

### 1. Stabilize current workflows and bug backlog

- Fix OCI and image accuracy issues that affect real-world registries
- Fix release and contributor workflow issues
- Continue to address long-running stability issues