Add changelog entry for IJ plugin removal

Author: dinomite

CHANGELOG.md

@@ -21,6 +21,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/).
 
 ### Fixed
 - fix tests which relied on a third party reporter in jcenter [#772](https://github.com/JLLeitschuh/ktlint-gradle/pull/772)
+- Remove IntelliJ plugin references from README [#775](https://github.com/JLLeitschuh/ktlint-gradle/pull/775)
 
 ## [12.1.1] - 2024-05-07