Add README documenting repository files and endpoints

README.md

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+# simple-server
+
+A simple Node.js/Express web server.
+
+## Installation
+
+```bash
+npm install
+```
+
+## Usage
+
+```bash
+npm start
+```
+
+The server will run on port **7000**.
+
+## File Descriptions
+
+| File | Description |
+|------|-------------|
+| `index.js` | Main entry point. Creates the Express app, registers routes, and starts the server on port 7000. |
+| `patchHandler.js` | Express router module that handles `PATCH /` requests. |
+| `test.txt` | A plain text file containing the numbers 1 through 20, one per line. |
+| `package.json` | Node.js project manifest. Defines dependencies (`express`, `nodemon`) and scripts (`start`). |
+| `package-lock.json` | Auto-generated lock file for npm, pinning exact dependency versions. |
+| `yarn.lock` | Auto-generated lock file for Yarn, pinning exact dependency versions. |
+| `.gitignore` | Specifies files and directories that Git should ignore. |
+
+## Endpoints
+
+| Method | Path | Description |
+|--------|------|-------------|
+| `GET`  | `/`  | Returns `Hello world!` |
+| `PATCH`| `/`  | Patch endpoint handled by `patchHandler.js` |