Add README for pow and powify plugins (#8225)

plugins/powify/README.md

@@ -0,0 +1,10 @@
+# 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)
+```