grunt: add README (#7198)

plugins/grunt/README.md

+# grunt plugin
+
+This plugin adds completions for [grunt](https://github.com/gruntjs/grunt).
+
+To use it, add `grunt` to the plugins array of your `.zshrc` file:
+```zsh
+plugins=(... grunt)
+```
+
+## Enable caching
+
+If you want to use the cache, set the following in your `.zshrc`:
+```zsh
+zstyle ':completion:*' use-cache yes
+```
+
+## Settings
+
+* Show grunt file path:
+  ```zsh
+  zstyle ':completion::complete:grunt::options:' show_grunt_path yes
+  ```
+* Cache expiration days (default: 7):
+  ```zsh
+  zstyle ':completion::complete:grunt::options:' expire 1
+  ```
+* Not update options cache if target gruntfile is changed.
+  ```zsh
+  zstyle ':completion::complete:grunt::options:' no_update_options yes
+  ```
+
+Note that if you change the zstyle settings, you should delete the cache file and restart zsh.
+
+```zsh
+$ rm ~/.zcompcache/grunt
+$ exec zsh
+```