[docs] Update installation docs to mention installation of Barracuda. (#3030)

Author: surfnerd

docs/Installation.md

@@ -33,6 +33,27 @@ The `UnitySDK` subdirectory contains the Unity Assets to add to your projects.
 It also contains many [example environments](Learning-Environment-Examples.md)
 to help you get started.
 
+### Package Installation
+
+If you intend to copy the `UnitySDK` folder in to your project, ensure that
+you have the [Barracuda preview package](https://docs.unity3d.com/Packages/com.unity.barracuda@0.3/manual/index.html) installed.
+
+To install the Barracuda package in Unity **2017.4.x**, you will have to copy the
+`UnityPackageManager` folder under the `UnitySDK` folder to the root directory of your
+project.
+
+To install the Barrcuda package in later versions of Unity, navigate to the Package
+Manager window by navigating to the menu `Window` -> `Package Manager`.  Click on the
+`Adavanced` dropdown menu to the left of the search bar and make sure "Show Preview Packages"
+is checked.  Search for or select the `Barracuda` package and install the latest version.
+
+<p align="center">
+  <img src="images/barracuda-package.png"
+       alt="Barracuda Package Manager"
+       width="710" border="10"
+       height="569" />
+</p>
+
 The `ml-agents` subdirectory contains a Python package which provides deep reinforcement
 learning trainers to use with Unity environments.
 

docs/Migrating.md

@@ -23,7 +23,7 @@ The versions can be found in
 * If you use RayPerception3D, replace it with RayPerceptionSensorComponent3D (and similarly for 2D). The settings, such as ray angles and detectable tags, are configured on the component now.
 RayPerception3D would contribute `(# of rays) * (# of tags + 2)` to the State Size in Behavior Parameters, but this is no longer necessary, so you should reduce the State Size by this amount.
 Making this change will require retraining your model, since the observations that RayPerceptionSensorComponent3D produces are different from the old behavior.
-* If you see messages such as `The type or namespace 'Barracuda' could not be found` or `The type or namespace 'Google' could not be found`, you will need to install the Barracuda preview package from the Package Manager.
+* If you see messages such as `The type or namespace 'Barracuda' could not be found` or `The type or namespace 'Google' could not be found`, you will need to [install the Barracuda preview package](Installation.md#package-installation).
 
 ## Migrating from ML-Agents toolkit v0.10 to v0.11.0