Update docs before milestone release 4.0.2

Encountered this issue after following instructions. saw issue sass/node-sass#3016 and recent fix facebook#9988

Author: JackHowa

Diff for: docusaurus/docs/adding-a-sass-stylesheet.md

@@ -10,12 +10,12 @@ Generally, we recommend that you don’t reuse the same CSS classes across diffe
 
 Following this rule often makes CSS preprocessors less useful, as features like mixins and nesting are replaced by component composition. You can, however, integrate a CSS preprocessor if you find it valuable.
 
-To use Sass, first install `node-sass`:
+To use Sass, first install `node-sass@4`:
 
 ```sh
-$ npm install node-sass --save
+$ npm install node-sass@4 --save
 $ # or
-$ yarn add node-sass
+$ yarn add node-sass@4
 ```
 
 Now you can rename `src/App.css` to `src/App.scss` and update `src/App.js` to import `src/App.scss`.