Document support for packaging projects

[kzu]

When creating non-trivial nugets with multiple projects included, the simple instructions in the current docs typically fall short and you need a proper packaging project instead.

This is already supported by nugetizer, but not documented at all. We need to document how a simple packaging project like:

<Project Sdk="Microsoft.Build.NoTargets/2.0.1" DefaultTargets="Pack">
  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
    <PackageId>MyPackage</PackageId>
    <Description>...</Description>
    <PackageTags>dotnet proxy runtime dynamic</PackageTags>
  </PropertyGroup>
  <ItemGroup>
     <!--  PackageReference/ProjectReference/Content/None/etc. items just work here too -->
  </ItemGroup>
</Project>

can be used to get very granular control of what's packed and where. The project needs to have a .msbuildproj extension.

May need multi-targeting feature from #89 to be a comprehensive solution.

[kzu]

Done now