Add README to NuGet package

[jpdillingham]

Per: https://devblogs.microsoft.com/nuget/add-a-readme-to-your-nuget-package/

This requires (at the time this issue was created) a preview version of the .NET SDK, which I'm not interested in using, so this will need to wait until this functionality has made it into a release.

The changes are:

  <ItemGroup>
    <AdditionalFiles Include=".\.CodeAnalysis\stylecop.json" Link="Properties\stylecop.json">
      <CopyToOutputDirectory>Never</CopyToOutputDirectory>
    </AdditionalFiles>
    <None Include="..\README.md" Pack="true" PackagePath="\" Link="Properties\README.md"/>
  </ItemGroup>

  <PropertyGroup>
    <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
    <Version>2.3.2</Version>
    <Authors>JP Dillingham</Authors>
    <Product>Soulseek.NET</Product>
    <PackageProjectUrl>https://github.com/jpdillingham/Soulseek.NET</PackageProjectUrl>
    <PackageLicenseExpression>GPL-3.0-or-later</PackageLicenseExpression>
    <Description>A .NET Standard client library for the Soulseek network.</Description>
    <Copyright>Copyright (c) JP Dillingham</Copyright>
    <RepositoryUrl>https://github.com/jpdillingham/Soulseek.NET</RepositoryUrl>
    <RepositoryType>git</RepositoryType>
    <PackageTags>dotnet csharp soulseek soulseek-network</PackageTags>
    <NeutralLanguage>en-US</NeutralLanguage>
    <PublishRepositoryUrl>true</PublishRepositoryUrl>
    <IncludeSymbols>true</IncludeSymbols>
    <SymbolPackageFormat>snupkg</SymbolPackageFormat>
    <EmbedUntrackedSources>true</EmbedUntrackedSources>
    <PackageReadmeFile>README.md</PackageReadmeFile>
  </PropertyGroup>