docs: add newly added invoice item service to changelog

Signed-off-by: Roald Nefs <info@roaldnefs.com>

Author: roaldnefs

CHANGELOG.md

@@ -5,5 +5,6 @@ All notable changes in **python-transip** are documented below.
 ### Added
 - This `CHANGELOG.md` file to be able to list all notable changes for each version of **python-transip**.
 - The `transip.v6.objects.ApiTestService` service to allow calling the test resource to make sure everything is working.
+- The `transip.v6.objects.InvoiceItemService` service to allow listing all invoice items on a `transip.v6.objects.Invoice` object.
 
 [Unreleased]: https://github.com/roaldnefs/python-transip/compare/v0.3.0...HEAD