Update README.md

Author: quantizor

README.md

@@ -12,7 +12,7 @@ This plugin is a highly recommended supplement to the base styled-components lib
 Install the plugin first:
 
 ```
-npm i -D babel-plugin-styled-components
+npm install --save-dev babel-plugin-styled-components
 ```
 
 Then add it to your babel configuration:
@@ -28,7 +28,7 @@ Then add it to your babel configuration:
 If you are using the styled-components v4 beta, you'll also need to use the beta version of this plugin:
 
 ```
-npm i -D babel-plugin-styled-components@beta
+npm install --save-dev babel-plugin-styled-components@beta
 ```
 
 Older versions should technically work, but some of the bundle size reduction benefits are only realized with the newer code!