Update requirements.txt with Usage example

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Updating dependencies in the YOLOv5 requirements.txt.

📊 Key Changes

• Clarified the purpose of the requirements.txt with a comment at the top.
• Removed unnecessary Cython comment since it's no longer a prerequisite directly handled in this file.
• Remaining dependencies unchanged.

🎯 Purpose & Impact

• To improve the clarity and maintenance of the requirements file by removing outdated information.
• This change helps users by making installation instructions simpler and more straightforward.
• No direct impact on YOLOv5 functionalities as no actual dependencies were added or removed.