docs: add deep-switch to community tools

[skyedolyn-sys]

What

Adds a new 社区工具 / Community Tools section to the README, listing deep-switch — a community-maintained menu-bar / tray companion for Deep Code.

Why

Several users hit ~/.deepcode/settings.json typos when switching providers by hand (e.g. #230 "Connection error" after editing config). deep-switch rewrites that file safely and lets users flip providers in one click, with no CLI restart. Listing it as a community tool gives those users an official-ish escape hatch without Deep Code taking on any maintenance burden — it's maintained by its own author.

Scope

• README.md only: one new section, one list item.
• Inserted between "获取帮助" and "协议" — no existing content modified.
• Bilingual (zh primary + en one-liner) to match the repo's voice.