add new article to the summary.md

Author: umbracotrd

16/umbraco-commerce/SUMMARY.md

@@ -47,6 +47,7 @@
 * [Use an Alternative Database for Umbraco Commerce Tables](how-to-guides/use-an-alternative-database-for-umbraco-commerce-tables.md)
 * [Customizing Templates](how-to-guides/customizing-templates.md)
 * [Configuring Cart Cleanup](how-to-guides/configuring-cart-cleanup.md)
+* [Configuring Abandoned Carts Notification](how-to-guides/configuring-abandoned-cart-notification.md)
 * [Limit Order Line Quantity](how-to-guides/limit-orderline-quantity.md)
 * [Implementing Product Bundles](how-to-guides/product-bundles.md)
 * [Implementing Member Based Pricing](how-to-guides/member-based-pricing.md)