Adding references to the Extensions package to help promote it. (#4967)

* Adding references to the Extensions package to help promote it.

* Updating header formatting to match the rest of the page

com.unity.ml-agents.extensions/Documentation~/com.unity.ml-agents.extensions.md

 ### Github via Package Manager
 In Unity 2019.4 or later, open the Package Manager, hit the "+" button, and select "Add package from git URL".
 
-![Package Manager git URL](https://github.com/Unity-Technologies/ml-agents/tree/release_12_docs/docs/images/unity_package_manager_git_url.png)
+![Package Manager git URL](https://github.com/Unity-Technologies/ml-agents/blob/release_12_docs/docs/images/unity_package_manager_git_url.png)
+
 In the dialog that appears, enter
  ```
  git+https://github.com/Unity-Technologies/ml-agents.git?path=com.unity.ml-agents.extensions

com.unity.ml-agents/Documentation~/com.unity.ml-agents.md

 # About ML-Agents package (`com.unity.ml-agents`)
 
-The Unity ML-Agents package contains the C# SDK for the [Unity ML-Agents
+The _ML-Agents_ package contains the primary C# SDK for the [Unity ML-Agents
 Toolkit].
 
 The package allows you to convert any Unity scene to into a learning environment
 instrumenting a Unity scene, setting it up for training, and then embedding the
 trained model back into your Unity scene. The machine learning algorithms that
 orchestrate training are part of the companion [Python package].
+
+Note that we also provide an _ML-Agents Extensions_ package
+(`com.unity.ml-agents.extensions`) that contains early/experimental features
+that you may find useful. This package is only available from the [ML-Agents
+GitHub repo].
 
 ## Package contents
 
 [execution order of event functions]: https://docs.unity3d.com/Manual/ExecutionOrder.html
 [connect with us]: https://github.com/Unity-Technologies/ml-agents#community-and-feedback
 [ml-agents forum]: https://forum.unity.com/forums/ml-agents.453/
+[ML-Agents GitHub repo]: https://github.com/Unity-Technologies/ml-agents/blob/release_12_docs/com.unity.ml-agents.extensions

docs/Installation.md

 
 - Unity package ([`com.unity.ml-agents`](../com.unity.ml-agents/)) contains the
   Unity C# SDK that will be integrated into your Unity scene.
+- Unity package
+  ([`com.unity.ml-agents.extensions`](../com.unity.ml-agents.extensions/))
+  contains experimental C#/Unity components that are not yet ready to be part
+  of the base `com.unity.ml-agents` package. `com.unity.ml-agents.extensions`
+  has a direct dependency on `com.unity.ml-agents`.
 - Three Python packages:
   - [`mlagents`](../ml-agents/) contains the machine learning algorithms that
     enables you to train behaviors in your Unity scene. Most users of ML-Agents
 - Install Python (3.6.1 or higher)
 - Clone this repository (Optional)
   - __Note:__ If you do not clone the repository, then you will not be
-  able to access the example environments and training configurations.
-  Additionally, the [Getting Started Guide](Getting-Started.md) assumes that
-  you have cloned the repository.
+  able to access the example environments and training configurations or the
+  `com.unity.ml-agents.extensions` package. Additionally, the
+  [Getting Started Guide](Getting-Started.md) assumes that you have cloned the
+  repository.
+- Install the `com.unity.ml-agents.extensions` Unity package (Optional)
 - Install the `mlagents` Python package
 
 ### Install **Unity 2018.4** or Later
 `com.unity.ml-agents` package
 [directly from the Package Manager registry](https://docs.unity3d.com/Manual/upm-ui-install.html).
 Please make sure you enable 'Preview Packages' in the 'Advanced' dropdown in
-order to find it.
+order to find the latest Preview release of the package.
 
 **NOTE:** If you do not see the ML-Agents package listed in the Package Manager
 please follow the [advanced installation instructions](#advanced-local-installation-for-development) below.
 
 If you are going to follow the examples from our documentation, you can open the
 `Project` folder in Unity and start tinkering immediately.
+
+### Install the `com.unity.ml-agents.extensions` Unity package (Optional)
+
+To install the `com.unity.ml-agents.extensions` package, you need to first
+clone the repo and then complete a local installation similar to what was
+outlined in the previous
+[Advanced: Local Installation for Development](#advanced-local-installation-for-development-1)
+section. Complete installation steps can be found in the
+[package documentation](../com.unity.ml-agents.extensions/Documentation~/com.unity.ml-agents.extensions.md#installation).
 
 ### Install the `mlagents` Python package
 

docs/Learning-Environment-Examples.md

     - Recommended minimum: 6
     - Recommended maximum: 20
 
-# Strikers Vs. Goalie
+## Strikers Vs. Goalie
 
 ![StrikersVsGoalie](images/strikersvsgoalie.png)