docs: add Simplified Chinese README

[whtis]

What

Add a Simplified Chinese translation of README.md as README.zh-CN.md, and a language switcher link at the top of both files.

Why

Codebuff has a growing audience among Chinese-speaking developers (the project ships with Qwen and DeepSeek as first-class model choices), but the entry point — the README — is English-only today. A native zh-CN README makes the project's positioning, multi-agent design, and SDK/CLI surface area much more accessible to that audience, with zero impact on existing readers.

Scope

• New file: README.zh-CN.md — a hand-written translation that preserves the original structure, headings, code blocks, links, and tone. Brand names (Codebuff, Freebuff, OpenRouter, etc.) and code samples are intentionally kept in their original form; only natural-language prose is translated.
• README.md: a single English | [简体中文](./README.zh-CN.md) line added below the title, matching the convention used by many bilingual READMEs on GitHub.

No code, build, or CI changes.

How tested

• Rendered both READMEs locally (markdown preview) — headings, code fences, images, and links all resolve correctly.
• Verified ./README.zh-CN.md link from the English README and ./README.md link back from the Chinese README both work.
• Diff is contained to two files (one new, one two-line edit); no other surfaces touched.

Happy to adjust phrasing, file naming (README_zh-CN.md vs README.zh-CN.md), or split the language switcher into a separate change if that fits the project better.

Drafted with AI assistance; verified by author before publishing.

[jahooma]

Oh wow, that's awesome! ChatGPT said it's a faithful translation. I'll merge it now, thanks!

[whtis]

Thanks so much for the quick merge — really happy to be contributing to this project! 🙌