Add README to cask plugin

plugins/cask/README.md

+# cask 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.