docs: contents read permission is required for private repos

### Summary

📚 https://github.com/slackapi/slack-health-score?tab=readme-ov-file#workflow-setup

The example setup steps assume the repo used is **public** which allows the checkout step to succeed without a specific `contents` permission.

But this fails for private repos!

```diff
  permissions:
    checks: write
+   contents: read
```

We ought to recommend setting `contents:read` here 🙏 ✨ 