Improve README with comprehensive challenges.json documentation

- Document all challenge fields including optional ones (testflag, hints, content)
- Explain hint system with sequential revelation and cost mechanism
- Detail Markdown and HTML support for descriptions and hints
- Document all dynamic placeholders with their purposes
- Add complete JSON example with proper structure
- Clarify required vs optional fields

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>