Added 404 page when a request does not match any routes.

client/app/components/NotFound.js

@@ -0,0 +1,10 @@
+// components/NotFound.js
+import React from 'react';
+
+const NotFound = () =>
+  <div>
+    <h3>404 page not found</h3>
+    <p>We are sorry but the page you are looking for does not exist.</p>
+  </div>
+
+export default NotFound;