Add more information to cask plugin README

plugins/cask/README.md

-# cask plugin
+# Cask plugin
 
-Loads `cask` completion from non-standard locations, such as if installed
+[Cask](https://github.com/cask/cask) is a project management tool for Emacs that helps
+automate the package development cycle; development, dependencies, testing, building,
+packaging and more.
+
+This plugin loads `cask` completion from non-standard locations, such as if installed
 via Homebrew or others. To enable it, add `cask` to your plugins array:
 
 ```zsh
 plugins=(... cask)
 ```
 
-Make sure you have the `cask` directory in your `$PATH` before loading
-Oh My Zsh, otherwise you'll get the "command not found" error.
+Make sure you have the `cask` directory in your `$PATH` before loading Oh My Zsh,
+otherwise you'll get a "command not found" error.