Add a note about xGPU sample being added into the main readme.
Add a note about xGPU sample being added into the main readme.

baesky/DirectX-Graphics-Samples,baesky/DirectX-Graphics-Samples,Microsoft/DirectX-Graphics-Samples,SuperWangKai/DirectX-Graphics-Samples,SuperWangKai/DirectX-Graphics-Samples,Microsoft/DirectX-Graphics-Samples,baesky/DirectX-Graphics-Samples,Microsoft/DirectX-Graphics-Samples,SuperWangKai/DirectX-Graphics-Samples