Importing React for the first example. (#14346)

Author: devrchancay

Diff for: packages/react-is/README.md

@@ -17,6 +17,7 @@ npm install react-is
 ### Determining if a Component is Valid
 
 ```js
+import React from "react";
 import * as ReactIs from "react-is";
 
 class ClassComponent extends React.Component {