new changelog and migration sections (#3841)

com.unity.ml-agents/CHANGELOG.md

 [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Major Changes
+### Minor Changes
+### Bug Fixes
+
+## [1.0.0-preview] - 2020-05-06
 
 ### Major Changes
 - Added new 3-joint Worm ragdoll environment. (#3798)

docs/Migrating.md

 
 # Migrating
 
-## Migrating from 0.15 to latest
+## Migrating from Release 1 to latest
+### Important changes
+### Steps to Migrate
+
+## Migrating from 0.15 to  Release 1
+
 
 ### Important changes