docs: add Coolify/Nixpacks self-hosting guide

[saadih]

Add a nixpacks.toml and accompanying readme section so users can self-host github-readme-stats on Coolify (or any other Nixpacks-based platform) via the Public Git Repository option.

The nixpacks.toml pins Node.js 22, installs all dependencies, and starts the Express server. Because dev dependencies aren't pruned, express (which lives in devDependencies) is available at runtime, so the existing package.json doesn't need to change.

This complements (rather than replaces) the existing On other platforms instructions, which remain accurate for users on platforms that prune dev deps and need to move express manually.

Files changed

• nixpacks.toml (new), 7 lines
• readme.md, adds an On Coolify via Nixpacks section plus a TOC entry

Tested locally

• npm ci --ignore-scripts installs cleanly (593 packages, no errors)
• node express.js boots on the configured PORT
• All 5 API routes (/api, /api/pin, /api/top-langs, /api/wakatime, /api/gist) respond with HTTP 200 and valid SVG

[vercel]

@saadih is attempting to deploy a commit to the github readme stats Team on Vercel.

A member of the Team first needs to authorize it.

[msndrbzyd-beep]

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