CHANGELOG.md update

Author: uerceg

CHANGELOG.md

@@ -1,3 +1,14 @@
+### Version 4.21.3 (4th May 2020)
+#### Fixed
+- Removed iAd timer from iOS native SDK.
+
+#### Native SDKs
+- [[email protected]][ios_sdk_v4.21.3]
+- [[email protected]][android_sdk_v4.21.1]
+- [[email protected]][windows_sdk_v4.17.0]
+
+---
+
 ### Version 4.21.2 (15th April 2020)
 #### Fixed
 - Added check for iOS timer source and block existence prior to starting it.