Add Marketplace badge

[glenn-jocher]

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🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced visibility of Ultralytics Actions on GitHub.

📊 Key Changes

• Added a badge link to the GitHub Actions Marketplace for Ultralytics Actions.

🎯 Purpose & Impact

• Purpose: To provide an easy access point for users to find and implement Ultralytics Actions from the GitHub Actions Marketplace.
• Impact: Improves discoverability of the tool, potentially increasing adoption and streamlining code quality management for more Ultralytics Python projects. 🚀