Update CHANGELOG.md

CHANGELOG.md

@@ -28,6 +28,9 @@ Use the following template:
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 ## [Unreleased][]
+### Changed
+*   Replace runtime reflection-based serialization using [gson](https://github.com/google/gson) with compile time
+    serializer deduction using [`kotlinx.serialization`](https://kotlinlang.org/docs/serialization.html).
 
 ## [2.0.4][] - 2020-06-05
 ### Fixed