Remove postcss dependency from usage install command

Closes #45

Author: Siilwyn

readme.md

@@ -6,7 +6,7 @@ Using [css-declaration-sorter](https://github.com/Siilwyn/css-declaration-sorter
 
 ## Usage
 Following the Prettier plugin guidelines, this package depends on Prettier as a peer dependency:  
-`npm install postcss prettier-plugin-css-order --save-dev`
+`npm install prettier-plugin-css-order --save-dev`
 
 To enable the plugin use the Prettier API, CLI or configuration file. For example using the JS configuration:
 ```js