[ios][android] Add a change log entry

platform/android/CHANGELOG.md

@@ -11,6 +11,9 @@ Mapbox welcomes participation and contributions from everyone.  If you'd like to
 ## 8.4.0-alpha.2 - September 11, 2019
 [Changes](https://github.com/mapbox/mapbox-gl-native/compare/android-v8.4.0-alpha.1...android-v8.4.0-alpha.2) since [Mapbox Maps SDK for Android v8.4.0](https://github.com/mapbox/mapbox-gl-native/releases/tag/android-v8.4.0-alpha.1): 
 
+### Performance improvements
+ - Newly loaded labels appear faster on the screen. [#15308](https://github.com/mapbox/mapbox-gl-native/pull/15308)
+
 ### Bug fixes
  - Fixed an issue of integer overflow when converting `tileCoordinates` to `LatLon`, which caused issues such as `queryRenderedFeatures` and `querySourceFeatures` returning incorrect coordinates at zoom levels 20 and higher. [#15560](https://github.com/mapbox/mapbox-gl-native/pull/15560)
 

platform/ios/CHANGELOG.md

@@ -2,6 +2,12 @@
 
 Mapbox welcomes participation and contributions from everyone. Please read [CONTRIBUTING.md](../../CONTRIBUTING.md) to get started.
 
+## master
+
+### Performance improvements
+
+* Newly loaded labels appear faster on the screen. ([#15308](https://github.com/mapbox/mapbox-gl-native/pull/15308))
+
 ## 5.4.0
 
 ### Styles and rendering