Add README for pow and powify plugins (#8225)

plugins/pow/README.md

+# pow plugin
+
+This plugin adds completion and commands for [pow](http://pow.cx/), a
+zero-configuration Rack server for macOS.
+
+To use it, add pow to the plugins array of your zshrc file:
+
+```sh
+plugins=(... pow)
+```
+
+## Commands
+
+- `kapow` will restart an app.
+
+  ```bash
+  kapow myapp
+  ```
+
+- `kaput` will show the standard output from any pow app.
+- `repow` will restart the pow process.

plugins/powify/README.md

+# powify plugin
+
+This plugin adds autocompletion for [powify](https://github.com/sethvargo/powify),
+an easy-to-use wrapper for Basecamp's [pow](https://github.com/basecamp/pow).
+
+To use it, add powify to the plugins array of your zshrc file:
+
+```sh
+plugins=(... powify)
+```