Add README with setup, run and test instructions

MIL-004 Documentation and release.

- Write README.md from the project template: overview, requirements,
  setup with a local .venv and pip upgrade, run, tests, AGPL license
  and links
- Document building the API docs with doxygen Doxyfile
- Verified: doxygen builds without warnings, and a clean copy installs,
  runs and passes the 13 tests following only the README

Closes #26
Closes #27
Closes #28

Task: MIL-004#1
Task: MIL-004#2
Task: MIL-004#3

README.md

@@ -1,2 +1,92 @@
-# 010-calc
+# 🧮 Calculator
 
+A beginner-friendly Python console program that adds, subtracts, multiplies and divides numbers, and lets you keep calculating with the previous result.
+
+## 📚 Table of Contents
+
+- [Overview](#-overview)
+- [Requirements](#-requirements)
+- [Setup](#-setup)
+- [Run](#-run)
+- [Tests](#-tests)
+- [License](#-license)
+- [Links](#-links)
+
+## 🧭 Overview
+
+Final project of Day 10 of Udemy's *100 Days of Code: The Complete Python Pro Bootcamp*. The program shows an ASCII calculator with the title beside it, then asks for a number, an operation (`+`, `-`, `*`, `/`) and a second number.
+
+The operations are functions stored in a dictionary (`OPERATIONS` in `src/calc/operations.py`) and called through it. After each result you can continue with it (`y`), start a new calculation (`n`, which calls `calculator()` again through recursion) or quit (`q`). Invalid input is asked for again, and dividing by zero starts over with a message.
+
+## 📋 Requirements
+
+- Python 3.13 or newer
+- No runtime dependencies
+- `pytest` for the tests (installed with the `dev` extra)
+- Optional: [Doxygen](https://www.doxygen.nl/) to build the API documentation
+
+## 🛠️ Setup
+
+Create a local virtual environment in the project folder:
+
+```bash
+python -m venv .venv
+```
+
+Activate it:
+
+```bash
+# Windows (PowerShell)
+.venv\Scripts\Activate.ps1
+
+# Windows (Git Bash)
+source .venv/Scripts/activate
+
+# Linux / macOS
+source .venv/bin/activate
+```
+
+Upgrade pip and install the project with the test tools:
+
+```bash
+python -m pip install --upgrade pip
+python -m pip install -e ".[dev]"
+```
+
+Leave the environment with `deactivate`. The `.env` file in the repository is for personal tooling only; the project never reads or imports it.
+
+## ▶️ Run
+
+With the environment active:
+
+```bash
+python -m calc
+```
+
+or, since the install adds a console script, simply:
+
+```bash
+calc
+```
+
+## 🧪 Tests
+
+```bash
+python -m pytest
+```
+
+Build the API documentation from the Doxygen comments (written to `docs/doxygen/`):
+
+```bash
+doxygen Doxyfile
+```
+
+## 📄 License
+
+GNU Affero General Public License v3.0, see [LICENSE](LICENSE).
+
+## 🔗 Links
+
+- [Repository](https://git.tirsystem.com/Tirsvad-Udemy-100_days_of_code)
+- [Documentation](docs/doxygen/html/index.html) (generated with `doxygen Doxyfile`)
+- [Issue tracker](https://git.tirsystem.com/Tirsvad-Udemy-100_days_of_code/010-calc/issues)