Add Discord Badge

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Update to Ultralytics xView YOLOv3 ReadMe and Evaluation Script.

📊 Key Changes

• Added a Discord badge with a link to the Ultralytics Discord community on the project's README.
• Inserted a blank line in the evaluation script for better readability.

🎯 Purpose & Impact

• Enhance Community Engagement: The Discord badge invites users to join the Ultralytics community for discussions, which can improve user support and engagement. 🤝
• Improve Code Quality: While a minor edit, the extra line in the evaluation script follows Python's style guidelines (PEP 8) for better code readability. 👀

The changes are minor, but they contribute to building a more supportive community and maintaining code quality standards. 🚀