Add Swoole v6.1.0 RC1 release notes documentation

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This PR adds comprehensive release notes for Swoole v6.1.0 RC1 in the new v6.1.md file. The documentation covers all major changes and improvements in this release candidate version.

Key sections included:

标准库扩展 (Standard Library Extensions)

• New PHP standard library syntax extensions that maintain full compatibility with official PHP syntax
• Object-oriented support for basic types (array/string/stream) with method chaining
• Typed arrays with type constraints for keys and values

✨ 新特性 (New Features)

• llHTTP parser replaces http_parser as default HTTP message parser for better performance
• Full coroutine support for file downloads in Swoole\Coroutine\Http\Client
• New iouring ftruncate support for extended async file operations

🐛 Bug 修复 (Bug Fixes)

• Thread mode optimizations including SWOOLE_THREAD hook_flags fixes
• Stability improvements for process management and resource cleanup
• Compatibility fixes for PHP 8.5 and various platform-specific issues

🛠️ 架构优化 (Architecture Optimizations)

• Achieved 86% unit test coverage milestone
• Refactored SSL, Socket, reactor, and string modules
• Performance improvements for multi-threading with independent memory management

废弃 (Deprecations)

• Removed support for select event mechanism in favor of poll
• Deprecated $waitEvent parameter in Swoole\Server::stop() method

The document follows the established repository conventions and provides complete Chinese documentation for developers and users tracking Swoole's development progress.

Fixes #94.

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