[added] Add module field to package json

Author: euxn23

package.json

@@ -3,6 +3,7 @@
   "version": "3.4.1",
   "description": "Accessible modal dialog component for React.JS",
   "main": "./lib/index.js",
+  "module": "./src/index.js",
   "repository": {
     "type": "git",
     "url": "https://github.com/reactjs/react-modal.git"