Rewrite README — make it actually readable

[tennisleng]

The old README was a generic bullet-point template that looked AI-generated. This rewrites it from scratch to read like a real person explaining their project:

• Explains what triangular arbitrage actually is and how the bot exploits it
• Walks through key design decisions (maker fees, BNB discount, dynamic sizing, slippage estimation) and why they matter
• Includes a full project structure map so someone can navigate the codebase
• Adds a config table with real defaults and descriptions
• Pulls in actual backtest numbers from backtest_results.json
• Adds a what I learned section with honest reflection on the challenges
• No emojis, no filler, no generic lists