pyenv: add README (#8224)

plugins/pyenv/README.md

+# pyenv 
+
+This plugin looks for [pyenv](https://github.com/pyenv/pyenv), a Simple Python version
+management system, and loads it if it's found. It also loads pyenv-virtualenv, a pyenv
+plugin to manage virtualenv, if it's found.
+
+To use it, add `pyenv` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... pyenv)
+```
+
+## Functions
+
+- `pyenv_prompt_info`: displays the Python version in use by pyenv; or the global Python
+  version, if pyenv wasn't found.