docs: add demo for React hooks and context (#150)

demo/reactjs/src/ClickButton.js

@@ -0,0 +1,15 @@
+import './ClickButton.css';
+import React from 'react';
+import { Context } from './Context';
+
+export function ClickButton() {
+  return (
+    <Context.Consumer>
+      {context => (
+        <button onClick={context.count}>
+          Click Here!
+        </button>
+      )}
+    </Context.Consumer>
+  );
+};