autojump: add README (#7253)

plugins/autoenv/README.md

+# Autojump plugin
+
+This plugin loads the [autojump navigation tool](https://github.com/wting/autojump).
+
+To use it, add `autojump` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... autojump)
+```
+
+More info on the usage: https://github.com/wting/autojump