Add change log entry

CHANGELOG.md

@@ -16,10 +16,18 @@
 
   This is a back porting from GL JS [#9333](https://github.com/mapbox/mapbox-gl-js/pull/9333)
 
+### ✨ New features
+
 - [core] Add Layer::serialize() method ([#16231](https://github.com/mapbox/mapbox-gl-native/pull/16231))
 
   New method allows serialization of a layer into a Value type, including all modifications done via runtime style API. New method is also an enabler for Style object serialization (sources, layers, etc).
 
+##### ⚠️  Breaking changes
+
+- [core] FileSource API improvements ([#16238](https://github.com/mapbox/mapbox-gl-native/pull/16238))
+
+  Changes to the `mbgl::FileSourceManager::getFileSource()` and `mbgl::OnlineFileSource` API.
+
 ## maps-v1.2.0 (2020.02-release-vanillashake)
 
 ### ✨ New features