docs: update according recommendation on migration postcss 8

Author: Scrum

Diff for: readme.md

@@ -13,7 +13,7 @@ Adds the ability to use a custom property in the options, at-rules.
 ## Install
 
 ```bash
-$ npm install postcss-at-rules-variables
+$ npm install --save-dev postcss postcss-at-rules-variables
 ```
 
 > **Note:** This project is compatible with node v10+