Updating changelog for 0.4.0 release (#76)

com.unity.perception/CHANGELOG.md

 ## [Unreleased]
 
 ### Added
+
+### Changed
+
+### Deprecated
+
+### Removed
+
+### Fixed
+
+## [0.4.0-preview.1] - 2020-08-07
+
+### Added
 Added new randomization tools
 
 Added support for 2020.1