Update README links for CSS modules and SASS (#5254)

Author: yuyokk

README.md

@@ -138,8 +138,9 @@ The [User Guide](https://github.com/facebook/create-react-app/blob/master/packag
 - [Importing a Component](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/README.md#importing-a-component)
 - [Code Splitting](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/README.md#code-splitting)
 - [Adding a Stylesheet](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/README.md#adding-a-stylesheet)
+- [Adding a Sass Stylesheet](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/README.md#adding-a-sass-stylesheet)
+- [Adding a CSS Modules Stylesheet](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/README.md#adding-a-css-modules-stylesheet)
 - [Post-Processing CSS](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/README.md#post-processing-css)
-- [Adding a CSS Preprocessor (Sass, Less etc.)](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/README.md#adding-a-css-preprocessor-sass-less-etc)
 - [Adding Images, Fonts, and Files](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/README.md#adding-images-fonts-and-files)
 - [Using the `public` Folder](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/README.md#using-the-public-folder)
 - [Using Global Variables](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/README.md#using-global-variables)