docs: add Simplified Chinese README

[whtis]

What

Adds README.zh-CN.md, a Simplified Chinese version of the main README, and a small language switcher (English · 简体中文) near the top of README.md so readers can find it.

Why

A meaningful share of the audience for AI memory / context-engine tools reads Chinese as their first language. A native zh-CN README lets them evaluate Supermemory without having to translate the pitch + API examples in their head. Several projects in this space already ship a README.zh-CN.md for the same reason.

Scope

• New file: README.zh-CN.md — full translation, mirrors the structure, anchors, code blocks, and image links of README.md.
• README.md: only the language switcher block was added; English content is unchanged.

No code, build config, or assets are touched.

Translation notes

• Kept code blocks and identifiers (client.add, containerTag, profile.static, etc.) verbatim — translating them would break the examples.
• Brand names left untranslated: Supermemory, Nova, MemoryBench, MCP, Claude Code, Cursor, OpenCode, OpenClaw, Hermes, etc.
• Headings translated such that the anchors used in the existing English README (#give-your-ai-memory, #build-with-supermemory-api) are mirrored to Chinese anchors inside the zh-CN file, so internal "Jump to..." links resolve correctly within each language.
• Kept the slightly informal, confident voice of the original rather than producing literal machine-translation output.

How tested

• Re-read end to end against the source README to check section-for-section parity.
• Verified internal anchor links inside README.zh-CN.md resolve to the corresponding zh-CN headings.
• Confirmed README.md change is purely additive (4 lines, the language switcher block).
• No build or test pipelines are affected by this PR.

Happy to adjust wording or split the language switcher into a separate PR if the maintainers prefer.

Drafted with AI assistance; verified by author before publishing.