Add SPI badges to README

[graycampbell]

📝 Summary

• Added Swift Package Index's Swift badge to README.md.
• Added Swift Package Index's platforms badge to README.md.

🛠️ Type of Change

• Bug fix (change that fixes an issue)
• New feature (change that adds functionality)
• Breaking change (bug fix or feature that is not backwards compatible)
• Documentation (DocC, API docs, markdown files, templates, etc.)
• Testing (new tests, updated tests, etc.)
• Refactoring or code formatting (no logic changes)
• Updating dependencies (Swift packages, Homebrew, etc.)
• CI/CD (change to automated workflows)

✅ Checklist

• I have added relevant tests
• I have verified all tests pass
• I have formatted my code using SwiftFormat
• I have updated documentation (if needed)
• I have added the appropriate label to my PR
• I have read the contributing guidelines
• I agree to follow this project's Code of Conduct