man: add README (#7276)

plugins/man/README.md

+# Man plugin
+
+This plugin adds a shortcut to insert man before the previous command.
+
+To use it, add `man` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... man)
+```
+# Keyboard Shortcuts
+| Shortcut                          | Description                                                            |
+|-----------------------------------|------------------------------------------------------------------------|
+| <kbd>Esc</kbd> + man              | add man before the previous command to see the manual for this command |