yii: add README (#8194)

plugins/yii/README.md

+# Yii plugin
+
+The plugin adds autocomplete commands and subcommands for [yii](https://www.yiiframework.com/).
+
+To use it, add `yii` to the plugins array of your zshrc file:
+```
+plugins=(... yii)
+```
+
+## Aliases
+
+| Alias  | Command              |
+|--------|----------------------|
+| yiic   | `protected/yiic`     |