[docs] Update installation instructions. (#3884)

Co-authored-by: Chris Elion <chris.elion@unity3d.com>

docs/Installation.md

 
 ### Install the `com.unity.ml-agents` Unity package
 
-The Unity ML-Agents C# SDK is a Unity Package. We are working on getting it added to the
-official Unity package registry which will enable you to install the `com.unity.ml-agents` package
-[directly from the registry](https://docs.unity3d.com/Manual/upm-ui-install.html) without cloning
-this repository. Until then, you will need to
-[install it from the local package](https://docs.unity3d.com/Manual/upm-ui-local.html) that you
-just cloned. You can add the `com.unity.ml-agents` package to
-your project by navigating to the menu `Window`  -> `Package Manager`. In the package manager
-window click on the `+` button. Select `Add package from disk...` and navigate into the
-`com.unity.ml-agents` folder and select the `package.json` folder.
+The Unity ML-Agents C# SDK is a Unity Package. You can install the `com.unity.ml-agents` package
+[directly from the Package Manager registry](https://docs.unity3d.com/Manual/upm-ui-install.html)
+without cloning this repository. Please make sure you enable 'Preview Packages'  in the 'Advanced'
+dropdown  in order to find it.
-**NOTE:** In Unity 2018.4 it's on the bottom right of the packages list, and in Unity 2019.3 it's
+If you want to try newer features from our development branch you
+can still [install the local package](https://docs.unity3d.com/Manual/upm-ui-local.html) from the
+repo that you just cloned.
+
+Add the local `com.unity.ml-agents` package to your project by:
+1. navigating to the menu `Window`  -> `Package Manager`.
+1. In the package manager window click on the `+` button.
+1. Select `Add package from disk...`
+1. Navigate into the `com.unity.ml-agents` folder.
+1. Select the `package.json` file.
+
+**NOTE:** In Unity 2018.4 the `+` button is on the bottom right of the packages list, and in Unity 2019.3 it's
 on the top left of the packages list.
 
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