autoenv: add README (#8184)

plugins/autoenv/README.md

+# Autoenv plugin
+
+This plugin loads the [Autoenv](https://github.com/inishchith/autoenv).
+
+To use it, add `autoenv` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... autoenv)
+```
+## Requirements
+
+In order to make this work, you will need to have the autoenv installed.
+
+More info on the usage and install: https://github.com/inishchith/autoenv