Editing documentation : Migrating.md (#2793)

* Editing documentation : Migrating.md

* Update docs/Migrating.md

Co-Authored-By: Jonathan Harper <jharper+moar@unity3d.com>

docs/Migrating.md

 #### Steps to Migrate
 * In order to be able to train, make sure both your ML-Agents Python package and UnitySDK code come from the v0.11 release. Training will not work, for example, if you update the ML-Agents Python package, and only update the API Version in UnitySDK.
 * If your Agents used visual observations, you must add a CameraSensorComponent corresponding to each old Camera in the Agent's camera list (and similarly for RenderTextures).
+* Since Brain ScriptableObjects have been removed, you will need to delete all the Brain ScriptableObjects from your `Assets` folder. Then, add a `Behavior Parameters` component to each `Agent` GameObject. You will then need to complete the fields on the new `Behavior Parameters` component with the BrainParameters of the old Brain.
 
 ## Migrating from ML-Agents toolkit v0.9 to v0.10