Add description for the username

Author: CompuIves

packages/dynamic-pages/pages/profile.js

@@ -1,3 +1,5 @@
 import React from 'react';
 
+// The now router will set a header based on the username that we can get from getInitialProps to know
+// which username to render. For now we can just stub the username.
 export default () => <div>Welcome to next.js!</div>;