更新 README.md 文件

[timebug]

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Revamps README.md to modernize setup and usage guidance for the Xiangqi engine/UI.

• Adds explicit Requirements (Python 2.7, Pygame 1.9+)
• Provides Linux/macOS steps to clone, compile C engine (make, make install), and run (python2 cchess.py), noting Python 2 requirement
• Reorganizes and formats Keyboard Shortcuts (keeps Space for new game)
• Removes outdated Windows-specific instructions and legacy links

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