update changelog with release date

Author: andrewthad

changelog.md

@@ -4,7 +4,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to the [Haskell Package Versioning Policy](https://pvp.haskell.org/).
 
-## [1.6.0] - 2019-??-??
+## [1.6.0] - 2019-09-30
 - Provide decode functions for decoding from `ShortText` and
  from `Bytes`. These two are implemented internally using
  the same function.