Merge pull request #3813 from wiwei/minsdk

Update the Getting Started page to mention the required windows SDK.

Author: cre8ivepark

Documentation/GettingStartedWithTheMRTK.md

@@ -11,6 +11,7 @@ To get started with the Mixed Reality Toolkit you will need:
 
 * [Unity 2018.2.13f1 +](https://unity3d.com/get-unity/download/archive)
 * [Latest MRTK release (Beta)](https://github.com/Microsoft/MixedRealityToolkit-Unity/releases)
+* [Windows SDK 18362+](https://www.microsoft.com/en-us/software-download/windowsinsiderpreviewSDK)
 * A dream
 
 ## Upgrading from the HoloToolkit (HTK)