Specified that <ButtonWidget> takes a 'prop' of 'caption' that is of …

…type 'string' and is required. From now on, not passing this prop will result in a runtime warning emitted to the JavaScript console.

webpack_in/entry.jsx

@@ -6,6 +6,7 @@
 console.log("JSX entry logic.");
 
 
+import PropTypes from 'prop-types';
 import React from 'react';
 import ReactDOM from 'react-dom';
 
@@ -29,6 +30,10 @@ class ButtonWidget extends React.Component {
       }
 }
 
+ButtonWidget.propTypes = {
+    caption:                    PropTypes.string.isRequired
+  };
+
 class HelloWidget extends React.Component {
     render() {
         return (