Update 0.5 release notes

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

@@ -17,3 +17,4 @@ Crystal 1.9, 1.10, and 1.11.
 #### Models and databases
 
 * A new [`#pks`](../../models-and-databases/reference/query-set.md#pks) method was introduced for query sets to make it easy to retrieve the primary key values of the model records targeted by a given current query set.
+* 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.