docs: add README.md file for project documentation

This request adds build, configuration and API documentation.

README.md

+
+# Waitlist Backend
+
+A lightweight, high-performance waitlist backend written in Go, using [Fiber v3](https://github.com/gofiber/fiber) as the web framework and [LevelDB](https://github.com/syndtr/goleveldb) for data storage. The backend supports email registration, rate limiting, and optional [hCaptcha](https://www.hcaptcha.com) verification for added security.
+
+## Features
+
+- **Email Registration:** Allows users to register their email for the waitlist, with each registration stored in LevelDB.
+- **In-Memory Waitlist Count:** Tracks the total number of registered emails with a quick-access in-memory counter.
+- **Rate Limiting:** Prevents abuse by limiting the number of registrations allowed from a single IP address within a specified time frame.
+- **Optional hCaptcha Verification:** Adds an optional layer of security using hCaptcha to prevent bots from registering.
+- **High Performance:** Built with Fiber v3, offering fast routing and efficient handling of concurrent requests.
+- **Lightweight Storage:** Uses LevelDB to store email and timestamp data, ensuring a low overhead for database operations.
+
+## Getting Started
+
+### Prerequisites
+
+- **Go 1.20+**
+- **Optional:** hCaptcha API Key (if enabling hCaptcha verification)
+
+### Installation
+
+1. **Clone the repository:**
+    ```bash
+    git clone https://git.randomchars.net/hizla/waitlist/backend.git
+    cd backend
+    ```
+
+2. **Build and run:**
+    ```bash
+    go build
+    ./backend
+    ```
+
+3. **Configure the environment: (Optional)**
+   - Create a `.env` file in the root directory and add the following configuration options:
+     ```env
+     DB=db
+     VERBOSE=1
+     HCAPTCHA_SECRET_KEY=unset # set your hCaptcha secret key
+     HCAPTCHA_SITE_KEY=unset # set your hCaptcha site key
+     LISTEN=unset
+     LISTEN_ADDR=127.0.0.1:3000
+     ALLOWED_URL=https://hizla.io
+     ```
+
+
+## Configuration Documentation
+
+| Name                | Usage                                            | Default          |
+|---------------------|--------------------------------------------------|------------------|
+| DB                  | Filesystem path to database. Can be relative.    | db               |
+| LISTEN              | Filesystem path to UNIX socket to listen on.     | unset            |
+| LISTEN_ADDR         | Address to listen on if LISTEN is unset.         | 127.0.0.1:3000   |
+| ALLOWED_URL         | Allowed URL enforced by CORS.                    | https://hizla.io |
+| HCAPTCHA_SITE_KEY   | hCaptcha site key. Disables hCaptcha if unset.   | unset            |
+| HCAPTCHA_SECRET_KEY | hCaptcha secret key. Disables hCaptcha if unset. | unset            |
+| VERBOSE             | Verbose logging. Set 0 to disable.               | 1                |
+
+
+## API Documentation
+
+### Routes
+
+#### 1. `POST /api/register`
+
+Registers a new email to the waitlist.
+
+- **Headers:**  
+  `Content-Type: application/json`
+
+- **Request Body:**
+    ```json
+    {
+      "email": "user@example.com",
+      "hcaptcha_response": "optional-hcaptcha-response"
+    }
+    ```
+
+- **Response:**
+    - `200 OK`: Registration successful.
+    - `400 Bad Request`: Invalid input data or hCaptcha validation failed.
+    - `429 Too Many Requests`: Rate limit exceeded.
+    - `500 Internal Server Error`: Server error.
+
+- **Rate Limiting:**  
+  Each IP address is limited to `5` registrations per week.
+
+#### 2. `GET /api/count`
+
+Returns the total number of registered emails in the waitlist.
+
+- **Response:**
+    ```
+    1234
+    ```
+ 
+## Contributing
+
+Contributions are welcome! Please follow these steps:
+
+1. Fork the repository.
+2. Create a new feature branch (\`git checkout -b feature/my-feature\`).
+3. Commit your changes (\`git commit -m "feat: add new feature"\`).
+4. Push the branch (\`git push origin feature/my-feature\`).
+5. Open a pull request.
+
+## License
+
+This project is licensed under the AGPLv3 License. See the [LICENSE](LICENSE) file for details.
+
+## Contact
+
+For any questions or support, feel free to open an issue.