Update changelog.

Author: nedtwigg

plugin-gradle/CHANGES.md

@@ -3,6 +3,8 @@
 We adhere to the [keepachangelog](https://keepachangelog.com/en/1.0.0/) format (starting after version `3.27.0`).
 
 ## [Unreleased]
+### Added
+* (spotless devs only) if you specify `-PspotlessModern=true` Spotless will run the in-progress Gradle `5.4+` code.  The `modernTest` build task runs our test suite in this way.  It will be weeks/months before this is recommended for end-users. ([#598](https://github.com/diffplug/spotless/pull/598))
 
 ## [4.2.1] - 2020-06-04
 ### Fixed