Link to new Sass doc

Author: gaearon

README.md

@@ -109,6 +109,7 @@ The [User Guide](https://github.com/facebookincubator/create-react-app/blob/mast
 - [Importing a Component](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md#importing-a-component)
 - [Adding a Stylesheet](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md#adding-a-stylesheet)
 - [Post-Processing CSS](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md#post-processing-css)
+- [Adding a CSS Preprocessor (Sass, Less etc.)](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md#adding-a-css-preprocessor-sass-less-etc)
 - [Adding Images and Fonts](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md#adding-images-and-fonts)
 - [Using the `public` Folder](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md#using-the-public-folder)
 - [Using Global Variables](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md#using-global-variables)