z: add markdown Readme (#8715)

plugins/z/README.md

@@ -0,0 +1,23 @@
+# z - jump around
+
+This plugin defines the [z command](https://github.com/rupa/z) that tracks your most visited directories and allows you to access them with very few keystrokes.
+
+### Example
+Assume that you have previously visited directory `~/.oh-my-zsh/plugins`. From any folder in your command line, you can quickly access it by using a regex match to this folder:
+
+```bash
+/usr/bin$ z plug  # Even 'z p' might suffice
+~/.oh-my-zsh/plugins$
+```
+
+### Setup
+To enable z, add `z` to your `plugins` array in your zshrc file:
+
+```zsh
+plugins=(... z)
+```
+
+### Further reading
+
+For advanced usage and details of z, see [README](./README) (in man page format, copied from [rupa/z](https://github.com/rupa/z)).
+