Update README.md with Discord badge

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

Adding a Discord badge to the README for enhanced community engagement.

Inserted a Discord badge link alongside the existing Ultralytics Actions badge in the README file.

Purpose: To foster a more dynamic and accessible community space where users and developers can easily join the Ultralytics Discord server for discussions, support, and networking.
Impact: This update should make it much easier for both new and existing members of the Ultralytics community to find and engage with each other on Discord, potentially leading to increased collaboration, faster problem-solving, and an overall more vibrant community ecosystem. ✨👥🔗