feat(docs): add README

Author: matoous

README.md

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+<div align="center">
+
+# SumUp Reader Examples
+
+[![Documentation][docs-badge]](https://developer.sumup.com)
+[![License](https://img.shields.io/github/license/sumup/sumup-reader-examples)](./LICENSE)
+
+</div>
+
+Learn how to pair SumUp card readers and start in-person payments using the SumUp SDKs. This repository contains backend examples for all our SDKs and a couple of frontend starters for managing readers and initiating checkouts.
+
+All you need to get started is an API key (if you don't have one yet, create it in the [SumUp Dashboard](https://me.sumup.com/settings/api-keys)) and your merchant code (which you can find in the [settings](https://me.sumup.com/settings)). We recommend starting with a Sandbox account that you can create in [the dashboard](https://me.sumup.com/settings/developer) to be able to test as many payments as you want without processing real money.
+
+Set your environment:
+
+```sh
+export SUMUP_API_KEY="your_api_key"
+export SUMUP_MERCHANT_CODE="your_merchant_code"
+```
+
+## Backend Examples
+
+Minimal servers that expose the following endpoints:
+
+- `POST /readers` – link (pair) a reader using a pairing code and name
+- `GET /readers` – list readers for the merchant
+- `POST /readers/{id}/checkout` – start a checkout on a reader for a given amount
+
+- **Node.js SDK** (https://github.com/sumup/sumup-ts)
+  - Runtime: [Node.js](./backend/node/)
+    ```sh
+    cd backend/node
+    npm install
+    npm start
+    ```
+  - Runtime: [Bun](./backend/bun/)
+    ```sh
+    cd backend/bun
+    bun install
+    bun run index.ts
+    ```
+  - Runtime: [Deno](./backend/deno/)
+    ```sh
+    cd backend/deno
+    deno run --allow-env --allow-net main.ts
+    ```
+
+- **Python SDK** (https://github.com/sumup/sumup-py)
+  ```sh
+  cd backend/python
+  uv run app.py
+  ```
+
+- **Java SDK** (https://github.com/sumup/sumup-java)
+  ```sh
+  cd backend/java
+  gradle run
+  ```
+
+- **Go SDK** (https://github.com/sumup/sumup-go)
+  ```sh
+  cd backend/go
+  go run .
+  ```
+
+- **Rust SDK** (https://github.com/sumup/sumup-rs)
+  ```sh
+  cd backend/rust
+  cargo run
+  ```
+
+- **.NET SDK** (https://github.com/sumup/sumup-dotnet)
+  ```sh
+  cd backend/dotnet
+  dotnet run
+  ```
+
+## Frontend Examples
+
+Both front-ends talk to the backend endpoints above. They let you pair a reader, list existing readers, and start a checkout with a chosen amount.
+
+- [Static](./frontend/static/)
+- [React](./frontend/react/)
+
+The UI also links to https://virtual-solo.sumup.com/ which provides a web-based card reader for testing.
+
+## Environment
+
+All backend examples use:
+
+```sh
+export SUMUP_API_KEY="your_api_key"
+export SUMUP_MERCHANT_CODE="your_merchant_code"
+```
+
+If the front-end is served from another origin, enable CORS on the backend or use a proxy.
+
+[docs-badge]: https://img.shields.io/badge/SumUp-documentation-white.svg?logo=data:image/svg+xml;base64,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