Split out 0.14 in the migration guide (#3374)

docs/Migrating.md

 
 # Migrating
 
+## Migrating from 0.14 to latest
+### Important changes
+### Steps to Migrate
-## Migrating from 0.13 to latest
+## Migrating from 0.13 to 0.14
 
 ### Important changes
 * The `UnitySDK` folder has been split into a Unity Package (`com.unity.ml-agents`) and an examples project (`Project`).  Please follow the [Installation Guide](Installation.md) to get up and running with this new repo structure.