Add Gizmos folder to Create New Environment guide (#1493)

* Add Gizmos folder to create new env guide

docs/Learning-Environment-Create-New.md

     but is the default as of 2018.3.)
 3. In a file system window, navigate to the folder containing your cloned
     ML-Agents repository.
-4. Drag the `ML-Agents` folder from `UnitySDK/Assets` to the Unity Editor
-    Project window.
+4. Drag the `ML-Agents` and `Gizmos` folders from `UnitySDK/Assets` to the Unity 
+    Editor Project window.
 
 Your Unity **Project** window should contain the following assets: