docs: update README.md

Author: un-def

README.md

@@ -1 +1,50 @@
 # virtualenv-pyenv
+
+A [virtualenv][virtualenv] Python discovery plugin using [pyenv][pyenv]
+
+## Installation
+
+```shell
+pip install virtualenv-pyenv
+```
+
+## Usage
+
+The Python discovery mechanism can be specified by:
+
+* the CLI option `--discovery`:
+  ```shell
+  virtualenv --discovery pyenv -p 3.10 testenv
+  ```
+
+* the environment variable `VIRTUALENV_DISCOVERY`:
+  ```shell
+  export VIRTUALENV_DISCOVERY=pyenv
+  virtualenv -p 3.10 testenv
+  ```
+
+* the [config][virtualenv-docs-config-file] option `discovery`:
+  ```ini
+  [virtualenv]
+  discovery = pyenv
+  ```
+
+  ```shell
+  virtualenv pyenv -p 3.10 testenv
+  ```
+
+The Python version can be expressed using either 2 or 3 version segments:
+
+* `-p 3.9`
+* `-p 3.9.3`
+
+In the former case, the latest version found will be used.
+
+## Limitations
+
+Only CPython is supported at the moment.
+
+
+[virtualenv]: https://virtualenv.pypa.io/
+[pyenv]: https://github.com/pyenv/pyenv
+[virtualenv-docs-config-file]: https://virtualenv.pypa.io/en/latest/cli_interface.html#configuration-file