Add readme.

guix_studio/build/vs_2019/msix_package_project/Readme.txt

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+The app packaging project can be used to generate the required app package upload file (.msixupload or .appxupload) for Windows Store Submission.
+
+To generate the GUIX Studio package upload file, run "build_guix_studio_msix_package.cmd" from "scripts" folder.
+
+Additional Resources:
+For more in-depth information on packaging UWP apps for Windows Store Submission, refer to Microsoft's official documentation at:
+https://learn.microsoft.com/en-us/windows/msix/package/packaging-uwp-apps