Include README on publish

[TheAngryByrd]

Is your feature request related to a problem? Please describe.

As of 5.0.300, you can package your README.md in your nuget package.

Describe the solution you'd like

The suggested solution from the blog post:

<Project Sdk="Microsoft.NET.Sdk">
    <PropertyGroup>
        ...
        <PackageReadmeFile>README.md</PackageReadmeFile>
        ...
    </PropertyGroup>

    <ItemGroup>
        ...
        <None Include="docs\README.md" Pack="true" PackagePath="\"/>
        ...
    </ItemGroup>
</Project>

Describe alternatives you've considered

n/a

Additional context

n/a

[TheAngryByrd]

Since this requires 5.0.300, this will be blocked from really working until fsprojects/FAKE#2597 is merged