Release v1.6.0 (#324)

* Update version and changelog for v1.6.0
* Update docs
diff --git a/docs/dyn/books_v1.notification.html b/docs/dyn/books_v1.notification.html
index 250a776..31252a0 100644
--- a/docs/dyn/books_v1.notification.html
+++ b/docs/dyn/books_v1.notification.html
@@ -102,6 +102,7 @@
       "A String",
     ],
     "reason": "A String",
+    "notificationGroup": "A String",
     "targetUrl": "A String",
     "notification_type": "A String",
     "pcampaign_id": "A String",