Fix build warnings "package ... is missing a readme"

[stakx]

No description provided.

[stakx]

It seems I somewhat misunderstood the purpose of <PackageReadmeFile>: The file referenced in that property will get rendered on https://nuget.org as a package summary, not when the package is installed. So the pre-existing buldscripts\readme.txt is not the right file to reference. Using the pre-existing README.md would be a closer fit, but still not optimal, since it focuses on contributing & development, not plain consumption. If we wanted to do this right, we'd probably need to write a third README.md targeting downstream users as the main audience. Closing for now.