docs: add Simplified Chinese translation of README (#3740)

Add README.zh-CN.md with a hand-crafted (non-machine) Chinese
translation of the project README, and add a language switcher
link at the top of both README files.

Code blocks, command names, badges, and links are kept identical to
the English source; only prose is translated.

Co-authored-by: yifosheng001 <yifosheng001@ke.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3 people

README.md

@@ -15,6 +15,11 @@
     <a href="https://github.github.io/spec-kit/"><img src="https://img.shields.io/badge/docs-GitHub_Pages-blue" alt="Documentation"/></a>
 </p>
 
+<p align="center">
+    <strong>English</strong> ·
+    <a href="./README.zh-CN.md">简体中文</a>
+</p>
+
 ---
 
 ## Table of Contents