Add Slack Forum badge to README

[glenn-jocher]

Add badge with link to join the new YOLOv5 Slack Forum!
https://join.slack.com/t/ultralytics/shared_invite/zt-w29ei8bp-jczz7QYUmDtgo6r6KcMIAg

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhancements to README aesthetics and community engagement links.

📊 Key Changes

• Improved alignment and spacing within README elements for better visual structure.
• Added a Slack channel invitation link for expanded community support and interaction.

🎯 Purpose & Impact

• 🎨 Aesthetics Improvement: Aligning visual elements creates a cleaner and more professional appearance for the project's landing page.
• 💬 Community Growth: Including a Slack invite provides users with a new platform for discussion, support, and collaboration, potentially accelerating problem-solving and innovation.