Complete README feature accuracy: Add comprehensive YAML support documentation

🎯 Accuracy Improvements:
- Enhanced UTC vs Standard Library comparison table
- Added YAML support row with 'Optional (with go-yaml)' clarification
- Added Text Encoding, Unix Timestamps, Day Boundaries, Debug Support rows
- Updated Dependencies to 'Zero deps (core)' for clarity

📝 Consistent Terminology:
- Changed 'JSON/Database ready' to 'Serialization ready' throughout
- Updated section title from 'JSON and Database operations' to 'Serialization and Database operations'
- Added YAML example alongside JSON in usage examples

🔧 Feature Documentation:
- Added YAML marshaling example with struct tags
- Updated auto-generated features list to include YAML support
- Enhanced database compatibility description
- Clarified optional vs core features

✅ Comprehensive Coverage:
- Now documents all serialization formats: JSON, YAML, Text, Database
- Accurately represents zero-dependency core with optional YAML
- Shows both required and optional dependencies clearly
- Maintains technical accuracy while highlighting advantages

These changes ensure the README comprehensively and accurately represents
all package capabilities without overselling or missing key features.

Author: jackspirou