dirhistory: add README (#7239)

plugins/dirhistory/README.md

+# Dirhistory plugin
+
+This plugin adds keyboard shortcuts for navigating directory history and hierarchy.
+
+To use it, add `dirhistory` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... dirhistory)
+```
+## Keyboard Shortcuts
+
+| Shortcut                          | Description                                               |
+|-----------------------------------|-----------------------------------------------------------|
+| <kbd>alt</kbd> + <kbd>left</kbd>  | Go to previous directory                                  |
+| <kbd>alt</kbd> + <kbd>right</kbd> | Undo <kbd>alt</kbd> + <kbd>left</kbd>                     |
+| <kbd>alt</kbd> + <kbd>up</kbd>    | Move into the parent directory                            |
+| <kbd>alt</kbd> + <kbd>down</kbd>  | Move into the first child directory by alphabetical order |