Update README.md

Author: optimalisatie

README.md

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 For an improved widget, see [📐 Style.Tools - Dev Tools for CSS Optimization](https://github.com/style-tools/browser-widget).
 
+We have created a custom Critical CSS Generator based on [PostCSS](https://github.com/postcss/postcss) that can provide a better result than [Penthouse.js](https://github.com/pocketjoso/penthouse). It supports multiple viewports which enables to extract production ready critical CSS for a responsive design with a single click.
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 ![image](https://user-images.githubusercontent.com/8843669/50854430-4aeaf100-1385-11e9-8f96-7d05619ed10a.png)
 
 ## About