NuGet package not copying files and transform files as expected - nuget

Question

I am trying to understand how NuGet packages work so that I can package some of my repeatedly used source for future projects.
I am working with a simple class library (.Net Foundation) and using the CLI to make the nuspec file and then packing from the CLI I get a nuget package that installs the Class Library DLL perfectly.
But now I want to add a transform to the App.Config that includes a connection string and I want to copy a simple readme type file to the destination project.
Using the following nuspec file the readme does not install to the readme folder as expected (doesn't install at all) and I can't get any of the app.config.transforms to work at all.
<?xml version="1.0"?>
<package xmlns="http://schemas.microsoft.com/packaging/2010/07/nuspec.xsd">
<metadata>
<id>$id$</id>
<version>$version$</version>
<title>$title$</title>
<authors>$author$</authors>
<owners>$author$</owners>
<requireLicenseAcceptance>false</requireLicenseAcceptance>
<description>$description$</description>
<releaseNotes>Stuff</releaseNotes>
<copyright>Copyright 2019</copyright>
<tags>123 1234</tags>
</metadata>
<files>
<file src="app.config.transform" target="content\app.config.transform" />
<file src="Model\State1.cs" target="content\State1.cs"/>
<file src="new.txt" target="content\new.txt"/>
</files>
</package>
Where should I put the transform files? It seems that they should be in a content folder in my source project but that isn't working. When I pack the nuget package I get an error about the files not being inside the content folder even though they clearly are.
With the above nuspec file I get an error:
Access to the path 'C:\Users\ UserName \.nuget\packages\MyDLLProject.lookups\1.1.7041.17482\content' is denied.
Why is that?