Add link to multi-gpu train tutorial

[NanoCode012]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced YOLOv5 documentation with a new Multi-GPU Training tutorial link.

📊 Key Changes

• Added a link to a new tutorial on Multi-GPU Training in the README file.

🎯 Purpose & Impact

• Purpose: To provide users with guidance on how to leverage multiple GPUs during model training, making training faster and more efficient for larger datasets or more complex models.
• Impact: Offers valuable resources for users looking to scale their training process, potentially attracting a broader audience who require multi-GPU setups and improving user experience for existing users.

[glenn-jocher]

Awesome!! Great work on the tutorial!