Add changelog entry for Key re-exprt

Author: phadej

changelog.md

@@ -4,6 +4,7 @@ For the latest version of this document, please see [https://github.com/haskell/
 
 - Change time instances of types with year (`Day`, `UTCTime`) to require years with at least 4 digits.
 - Change `KeyValue` instances to be more general (and use equality to constraint them) instead of being more lax flexible instances.
+- Export `Key` type also from `Data.Aeson.KeyMap` module
 
 ### 2.0.3.0