pod: add README (#8257)

plugins/pod/README.md

+# pod
+
+This plugin adds completion for [`CocoaPods`](https://cocoapods.org/).
+CocoaPods is a dependency manager for Swift and Objective-C Cocoa projects.
+
+To use it, add `pod` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... pod)
+```