adding NuGet logo and README support

[Aaronontheweb]

Working on trying to implement https://devblogs.microsoft.com/nuget/add-a-readme-to-your-nuget-package/ but my MSBuild-foo is not yet strong enough....

[Aaronontheweb]

NuGet package stage worked on the build server. I'll have to retry doing that locally.

[Aaronontheweb]

Looks like the build system fails on my machine due to NuGet/Home#10791 - can't include the README file with our NuGet package until snupkg adds support for it. Otherwise, we lose SourceLink support - which is more useful and valuable than a README in the NuGet package.

[Aaronontheweb]

This doesn't fail on the build server because it uses an older version of the .NET 5 SDK than I do on my development machine.

Server: 5.0.101
My machine: 5.0.300

[Aaronontheweb]

In theory, I could split the snupkg and nupkg publication into two different discrete steps and manually pass in a flag to disable the PackReadmeFile variable but I think I'd rather just wait for a fix.

[Aaronontheweb]

NuGet/Home#10791 (comment) - need to upgrade to the .NET 6.0 runtime to resolve this issue.

[Aaronontheweb]

For reasons I don't fully understand, NuGet pack now appears to be working....

[Aaronontheweb]

Let's test this out now that we've merged in #5373

[Aaronontheweb]

This now works properly on .NET 6! Going to update our README.md to be a little more NuGet-friendly in a subsequent issue.