Add storybook story for DropdownMenu component

packages/components/src/dropdown-menu/stories/index.js

@@ -0,0 +1,27 @@
+/**
+ * External dependencies
+ */
+import { text, object } from '@storybook/addon-knobs';
+
+/**
+ * Internal dependencies
+ */
+import DropdownMenu from '../';
+
+export default { title: 'Components/DropdownMenu', component: DropdownMenu };
+
+export const _default = () => {
+	const label = text( 'Label', 'Select a direction.' );
+	const icon = text( 'Icon', 'ellipsis' );
+	const controls = object( 'Controls', [
+		{
+			title: 'Up',
+			icon: 'arrow-up',
+		},
+		{
+			title: 'Down',
+			icon: 'arrow-down',
+		},
+	] );
+	return <DropdownMenu icon={ icon } label={ label } controls={ controls } />;
+};