Add actionlint instructions to CONTRIBUTING.md

Add a short section to `CONTRIBUTING.md` showing how to run `actionlint` locally to mirror the CI job.

Example snippet:
```markdown
### Linting GitHub Actions

Run actionlint locally using Docker:
\`\`\`bash
docker run --rm -v "$(pwd):/repo" -w /repo ghcr.io/rhysd/actionlint:latest -color
\`\`\`
```