Update changelog for release 8 (#4548)

* update changelog before release 8

com.unity.ml-agents/CHANGELOG.md

 [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+
+### Major Changes
+#### com.unity.ml-agents (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
+
+### Minor Changes
+#### com.unity.ml-agents (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
+
+### Bug Fixes
+#### com.unity.ml-agents (C#)
+#### ml-agents / ml-agents-envs / gym-unity (Python)
+
+
+## [1.5.0-preview] - 2020-10-14
- yaml configuration file. [More information here](https://github.com/Unity-Technologies/ml-agents/blob/master/docs/Training-Configuration-File.md#rnd-intrinsic-reward)
+ yaml configuration file. [More information here](https://github.com/Unity-Technologies/ml-agents/blob/master/docs/Training-Configuration-File.md#rnd-intrinsic-reward) (#4473)
- - Stacking for compressed observations is now supported. An addtional setting
+ - Stacking for compressed observations is now supported. An additional setting
- - Added new visual 3DBall environment. (#4513)
+ - Added a new visual 3DBall environment. (#4513)
- - In `VisualFoodCollector` scene, a vector flag representing the frozen state of
+ - In the `VisualFoodCollector` scene, a vector flag representing the frozen state of
+ - Added conversion to string for sampler classes to increase the verbosity of
+ the curriculum lesson changes. The lesson updates would now output the sampler
+ stats in addition to the lesson and parameter name to the console.  (#4484)
+ - Localized documentation in Russian is added. Thanks to @SergeyMatrosov for
+ the contribution. (#4529)
- - Fixed a bug where accessing the Academy outside of play mode would cause the Academy to get stepped multiple times when in play mode. (#4532)
+ - Fixed a bug where accessing the Academy outside of play mode would cause the
+ Academy to get stepped multiple times when in play mode. (#4532)
 #### ml-agents / ml-agents-envs / gym-unity (Python)