Add prototype banner warning

source/javascripts/application.js

@@ -1,2 +1,3 @@
 //= require govuk_tech_docs
 //= require ./copy.js
+//= require ./prototype-banner.js

source/javascripts/prototype-banner.js

@@ -0,0 +1,24 @@
+(function () {
+  const h1 = document.querySelector('h1');
+
+  const banner = document.createElement('div');
+
+  banner.innerHTML =
+'<div class="govuk-notification-banner" role="region" aria-labelledby="prototype-banner-title">' +
+  '<div class="govuk-notification-banner__header">' +
+    '<h2 class="govuk-notification-banner__title" id="prototype-banner-title">' +
+      'Warning' +
+    '</h2>' +
+  '</div>' +
+  '<div class="govuk-notification-banner__content">' +
+    '<p class="govuk-notification-banner__heading">' +
+      'This documentation is under development and should not be used.' +
+    '</p>' +
+    '<p>If you\'re interested in using the API in the future, please contact ' +
+      '<a href="mailto:[email protected]">[email protected]</a>.' +
+    '</p>' +
+  '</div>' +
+'</div>';
+
+  h1.insertAdjacentElement('afterend', banner);
+})();

source/stylesheets/screen.css.scss

@@ -1,4 +1,5 @@
 @import "govuk_tech_docs";
+@import "govuk/components/notification-banner/index";
 @import "govuk/components/tabs/index";
 
 @import "./app";