Add storybook story for toolbar button component

packages/components/src/toolbar-button/stories/index.js

@@ -0,0 +1,23 @@
+/**
+ * External dependencies
+ */
+import { text } from '@storybook/addon-knobs';
+
+/**
+ * Internal dependencies
+ */
+import { Toolbar } from '../../';
+import ToolbarButton from '../';
+
+export default { title: 'Components/ToolbarButton', component: ToolbarButton };
+
+export const _default = () => {
+	const label = text( 'Label', 'This is an example label.' );
+	const icon = text( 'Icon', 'wordpress' );
+
+	return (
+		<Toolbar __experimentalAccessibilityLabel="Example Toolbar">
+			<ToolbarButton icon={ icon } label={ label } />
+		</Toolbar>
+	);
+};