📚 docs: Update README

README.md

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+# IP Test
+
+🌐 Minimal HTTP server that echoes your IP and request headers as HTML or JSON.
+
+Built with [Elysia](https://elysiajs.com/) on [Bun](https://bun.sh/).
+
+## Usage
+
+Send a request to `/`:
+
+- Browsers (`Accept: text/html` or `*/*`) get an HTML page listing the detected IP and all request headers.
+- Anything requesting `Accept: application/json` gets a JSON body: `{ "ip": "...", "headers": { ... } }`.
+
+```bash
+curl -H "Accept: application/json" http://localhost:3000/
+```
+
+## Development
+
+Requires [Bun](https://bun.sh/) 1.2+.
+
+```bash
+bun install
+bun run dev      # hot-reload dev server
+bun run start    # start the server
+bun test         # run tests
+bun run check    # typecheck + lint/format check + tests
+```
+
+## Docker
+
+```bash
+docker build -t ip-test .
+docker run -p 3000:3000 ip-test
+```
+
+## License
+
+[MIT](LICENSE)