Update Installation.md (#4013)

* Update Installation.md

Made it clear on implications for new users on NOT cloning the repo during installation

* Update Installation.md

* Removed trailing whitespace

docs/Installation.md

 - Install Unity (2018.4 or later)
 - 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.
 - Install the `com.unity.ml-agents` Unity package
 - Install the `mlagents` Python package