Updating the Changelog and Migrating (#3620)

com.unity.ml-agents/CHANGELOG.md

 
 ## [Unreleased]
 ### Major Changes
+
+### Minor Changes
+
+
+## [0.15.0-preview] - 2020-03-18
+### Major Changes
  - `Agent.CollectObservations` now takes a VectorSensor argument. (#3352, #3389)
  - Added `Agent.CollectDiscreteActionMasks` virtual method with a `DiscreteActionMasker` argument to specify which discrete actions are unavailable to the Agent. (#3525)
  - Beta support for ONNX export was added. If the `tf2onnx` python package is installed, models will be saved to `.onnx` as well as `.nn` format.

docs/Migrating.md

 
 # Migrating
 
-## Migrating from 0.14 to latest
+## Migrating from 0.15 to latest
+
+### Important changes
+
+### Steps to Migrate
+
+
+## Migrating from 0.14 to 0.15
 
 ### Important changes
 * The `Agent.CollectObservations()` virtual method now takes as input a `VectorSensor` sensor as argument. The `Agent.AddVectorObs()` methods were removed.