Add the README to explain the usage of OpenXR version of the sample.

[yl-msft]

Description

Name of sample: (example: ResizeAppView, or proposed name of new sample)

Testing

Type of change

• [x] Bug fix
• [ ] New feature
• [ ] Porting to new language

Supported platforms

Minimum OS version: (example: 18362)

• [ ] All UWP platforms
• [ ] Desktop
• [x] Holographic
• [ ] IoT
• [ ] Xbox
• [ ] 10X

Supported languages

• [x] C#
• [ ] C++/WinRT

Additional remarks