更新 README.md

[timebug]

Auto-generated by AI Agent - Session ID: 694d6fb368002ace2e5a3370

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Major README overhaul for clarity and onboarding.

• Adds project overview, feature list (UCCI engine, Python/Pygame GUI, AI/Network modes)
• Documents prerequisites and build/run steps (make install, run pycchess/cchess.py)
• Introduces basic controls and GPLv3 licensing info
• Removes outdated OS-specific install notes and legacy Windows download instructions

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