update changelog

Author: mikeebowen

CHANGELOG.md

@@ -4,6 +4,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## [2.18.0] 2022-xx-xx
+
+### Added
+- Added DocumentFormat.OpenXml.Office.SpreadSheetML.Y2021.ExtLinks2021 namespace
+- Added durableId attribute to DocumentFormat.OpenXml.Wordprocessing.NumberingPictureBullet
+
 ## [2.17.1] - 2022-06-28
 
 ### Removed