Add "optional" translation

Jeroen · web-flow · commit 221c593768ef · 2017-10-16T17:13:40.000+02:00
diff --git a/app/code/Magento/Checkout/i18n/en_US.csv b/app/code/Magento/Checkout/i18n/en_US.csv
@@ -176,3 +176,4 @@ Payment,Payment
 "Not yet calculated","Not yet calculated"
 "We received your order!","We received your order!"
 "Thank you for your purchase!","Thank you for your purchase!"
+"optional", "optional"
