Update README.MD

Microsoft June 2021 GDK LKG Private Test Build [19041.6947.210524-2100]

2106.GDK Private Test Build

This is a private test build of the PUBLIC version of the GDK which contains the tools, binaries, and runtimes required to build games for the Xbox App on Windows 10.

**Does not contain the Xbox Extensions**

**Files Updated with GDK vs. PGDK Branding**

 - EULA (LICENSE_EN-US.MD)
 - INSTALLER SPLASH SCREEN

No other changes except 2106 features, updates and fixes.

v2021.03.21 (Final)