Update 0.5 release notes

docs/docs/the-marten-project/release-notes/0.5.md

@@ -25,6 +25,7 @@ _Under development._
 * A [`#count`](pathname:///api/dev/Marten/DB/Model/Querying/ClassMethods.html#count(field%3AString|Symbol|Nil%3Dnil)-instance-method) method is now available on model classes and provides the same functionality as the [`#count`](../../models-and-databases/reference/query-set.md#count) query set method.
 * A new [`#bulk_create`](../../models-and-databases/reference/query-set.md#bulk_create) method was introduced to make it easy to insert multiple model instances into the database in a single query (which can be useful when dealing with large amounts of data that need to be inserted into the database).
 * A new [`#average`](../../models-and-databases/reference/query-set.md#average) method was introduced to allow computing the average values of a specific model field at the database level for the records targeted by a specific query set.
+* A new [`#sum`](../../models-and-databases/reference/query-set.md#sum) method was introduced to allow computing the sum of the values of a specific model field at the database level for the records targeted by a specific query set.
 
 #### Handlers and HTTP