bower: add README (#7216)

plugins/bower/README.md

+# Bower plugin
+
+This plugin adds completion for [Bower](https://bower.io/) and a few useful aliases for common Bower commands.
+
+To use it, add `bower` to the plugins array in your zshrc file:
+
+```
+plugins=(... bower)
+```
+
+## Aliases
+
+| Alias | Command         | Description                                            |
+|-------|-----------------|--------------------------------------------------------|
+| bi    | `bower install` | Installs the project dependencies listed in bower.json |
+| bl    | `bower list`    | List local packages and possible updates               |
+| bs    | `bower search`  | Finds all packages or a specific package.              |
+