📝 docs: add package description and usage to README (#46)

Author: gaborbernat

README.md

@@ -1,8 +1,43 @@
 # [`python-discovery`](https://python-discovery.readthedocs.io/en/latest/)
 
 [![PyPI](https://img.shields.io/pypi/v/python-discovery?style=flat-square)](https://pypi.org/project/python-discovery/)
-[![Supported Python
-versions](https://img.shields.io/pypi/pyversions/python-discovery.svg)](https://pypi.org/project/python-discovery/)
+[![Supported Python versions](https://img.shields.io/pypi/pyversions/python-discovery.svg)](https://pypi.org/project/python-discovery/)
 [![Downloads](https://static.pepy.tech/badge/python-discovery/month)](https://pepy.tech/project/python-discovery)
 [![check](https://github.com/tox-dev/python-discovery/actions/workflows/check.yaml/badge.svg)](https://github.com/tox-dev/python-discovery/actions/workflows/check.yaml)
 [![Documentation Status](https://readthedocs.org/projects/python-discovery/badge/?version=latest)](https://python-discovery.readthedocs.io/en/latest/?badge=latest)
+
+## What is python-discovery?
+
+`python-discovery` is a library for discovering Python interpreters installed on your machine. You may have multiple
+Python versions from system packages, [pyenv](https://github.com/pyenv/pyenv), [mise](https://mise.jdx.dev/),
+[asdf](https://asdf-vm.com/), [uv](https://docs.astral.sh/uv/), or the Windows registry (PEP 514). This library finds
+the right one for you.
+
+Give it a requirement like `python3.12` or `>=3.11,<3.13`, and it searches all known locations, verifies each candidate,
+and returns detailed metadata about the match. Results are cached to disk so repeated lookups are fast.
+
+## Usage
+
+```python
+from pathlib import Path
+
+from python_discovery import DiskCache, get_interpreter
+
+cache = DiskCache(root=Path("~/.cache/python-discovery").expanduser())
+result = get_interpreter("python3.12", cache=cache)
+if result is not None:
+    print(result.executable)  # /usr/bin/python3.12
+    print(result.implementation)  # CPython
+    print(result.version_info[:3])  # (3, 12, 1)
+```
+
+The `get_interpreter()` function accepts various specification formats:
+
+- Absolute path: `/usr/bin/python3.12`
+- Version: `3.12`
+- Implementation prefix: `cpython3.12`
+- PEP 440 specifier: `>=3.10`, `>=3.11,<3.13`
+
+## Documentation
+
+Full documentation is available at [python-discovery.readthedocs.io](https://python-discovery.readthedocs.io/en/latest/)