docs: add README with endpoint docs and development guide

Author: fengmk2

README.md

@@ -0,0 +1,46 @@
+# vp-setup-exe-downloader
+
+Download redirector for the Vite+ Windows installer. Auto-detects CPU architecture and redirects to the correct GitHub release asset.
+
+## Endpoints
+
+### `GET /`
+
+Redirects to the [Vite+ installation guide](https://viteplus.dev/guide/#install-vp).
+
+### `GET /download`
+
+Redirects to the latest `vp-setup.exe` GitHub release asset.
+
+**Query parameters:**
+
+| Param  | Description                                      | Example                |
+| ------ | ------------------------------------------------ | ---------------------- |
+| `arch` | Override architecture detection (`x64`, `arm64`) | `?arch=arm64`          |
+| `tag`  | Pin to a specific release tag                    | `?tag=v0.1.17-alpha.0` |
+
+**Architecture detection:**
+
+1. `?arch=` query parameter (explicit override)
+2. `User-Agent` header (detects `ARM64`/`aarch64`)
+3. Defaults to `x64`
+
+## Development
+
+```bash
+vp install
+vp dev        # start dev server
+vp check      # format, lint, type check
+vp test       # run tests
+```
+
+## Deployment
+
+Deployed to [Cloudflare Workers](https://workers.cloudflare.com/) via [Void](https://void.cloud). Pushes to `main` trigger automatic deployment.
+
+### Environment variables
+
+| Variable       | Required | Description                                                |
+| -------------- | -------- | ---------------------------------------------------------- |
+| `GITHUB_TOKEN` | No       | GitHub token for higher API rate limits (60/hr -> 5000/hr) |
+| `VOID_TOKEN`   | Yes (CI) | Void deployment token                                      |