Rewrite README with current workflow and badges

## Summary

README accurately describes the current build / contribute / release workflow, drops Travis-era references, and surfaces Hackage / CI / license badges.

## Motivation

Current README references Travis ("utilizes travis to automatically check pull requests") and git flow ("This project follows git flow"), neither of which will be true after this milestone. There are no badges, so a Hackage visitor landing on the GitHub repo cannot see at a glance whether the package is alive, what version is current, or what GHC versions are supported. The Hackage link in the existing README points at the wrong package (`collection-json` instead of `siren-json`).

## Scope

- Apply the `readme` skill.
- Add badges (shields.io): Hackage version, Hackage dependencies, CI status (from #63), license, GHC versions supported.
- Replace "Reporting Issues" / "Contributors" sections with current guidance: link to the issue tracker, contribution flow (squash PRs to `main`), `COPYRIGHT` attribution.
- Drop the "git flow" reference.
- Drop the Travis link.
- Add a brief usage example (decoding/encoding a small Siren entity) — currently the README only links to Hackage docs.
- Fix the broken Hackage link (currently `hackage.haskell.org/package/collection-json`).

## Acceptance criteria

- [ ] No references to `develop`, `master`, `git flow`, or `travis` in the README.
- [ ] Hackage version badge renders the current version.
- [ ] CI badge renders the latest workflow status.
- [ ] At least one runnable code example (decode or encode).
- [ ] Hackage link points at `siren-json`.

## Additional context

Pulls badge URLs from #63 (CI workflow) and #64 (release workflow) once those exist.