Nuget package description

[jdh30]

This package is described on Nuget as "Elm-inspired encoder and decoder for JSON". I was surprised to find that this is the wrong package if you want to use Thoth.Json on .NET Core or the .NET Framework (you must use Thoth.Json.Net).

Would be great if the description of this package on Nuget made it clear that it isn't intended for .NET or what it is intended for instead.

[MangelMaxime]

I added a note about runtime support in the description.

Thank you for the proposition :)