Update documentation by my suggestion in #889

Noticing cdn install in docs because I'd to check <ReactModal> tag in docs example but didn't see this in Installation section :)

Author: EliseyMartynov

docs/index.md

@@ -11,7 +11,18 @@ To install the stable version you can use [npm](https://npmjs.org/) or [yarn](ht
 
     $ npm install react-modal
     $ yarn add react-modal
-
+    
+To install react-modal in React CDN app:
+
+   - Add this CDN script tag after React CDN scripts and before your JS files (for example from [cdnjs](https://cdnjs.com/)): 
+    
+            <script src="https://cdnjs.cloudflare.com/ajax/libs/react-modal/3.14.3/react-modal.min.js"
+            integrity="sha512-MY2jfK3DBnVzdS2V8MXo5lRtr0mNRroUI9hoLVv2/yL3vrJTam3VzASuKQ96fLEpyYIT4a8o7YgtUs5lPjiLVQ=="
+            crossorigin="anonymous"
+            referrerpolicy="no-referrer"></script>
+    
+   - Use `<ReactModal>` tag inside your React CDN app.
+    
 
 ## [General Usage](#usage)