Add reference to the nightly nuget source in README.md

[ArchercatNEO]

No description provided.

[GeorgeS2019]

What is stopping this PR to merge?

[raulsntos]

Thanks for the contribution! I'm a bit hesitant to encourage the use of nightly builds too much, specially since I expect the workflow to change a lot when the new dotnet module is available.

For example, the workflow suggested in this PR mentions referencing packages directly instead of using the Godot.NET.Sdk which is what most projects will use (like in GodotSharp). Godot.NET.Sdk already includes the package references, so adding <PackageReference> won't work out of the box.