Update changelog for v3.6 release

Author: tomdalling

CHANGELOG.md

@@ -5,7 +5,7 @@ Notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## Unreleased
+## [3.6.0] - 2020-09-01
 ### Added
 - `RangeOf` built-in validator, for validating `Range` objects
 - `HashCoercer` built-in coercer for homogeneous `Hash` objects