Update changelog.

Author: nedtwigg

CHANGELOG.md

@@ -5,6 +5,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+- New plugin [`com.diffplug.if-git-diff`](IF_GIT_DIFF.md) ([#31](https://github.com/diffplug/spotless-changelog/pull/31)).
 
 ## [2.3.2] - 2021-11-29
 ### Fixed