[chore] update installation section.

Author: diasbruno

README.md

@@ -25,8 +25,16 @@ Accessible modal dialog component for React.JS
 
 To install the stable version:
 
+With [npm](https://npmjs.org/) installed, run:
+
+```
+$ npm install react-modal
+```
+
+...with [yarn](https://yarnpkg.com):
+
 ```
-npm install --save react-modal
+$ yarn add react-modal
 ```
 
 ## Usage