Can you add package to support InAttribute for netstandard1.0 as target

[AlexRadch]

When I use ref readonly and in parameter in package for netstandard1.0 target, compiler return error Predefined type 'System.Runtime.InteropServices.InAttribute' is not defined or imported.

Can you add new package like Nullable to support ref readonly and in parameter for netstandard1.0.

Thanks.

[manuelroemer]

Hey, sorry to disappoint, but that's something that I'm not going to tackle because I don't have the time/personal need for it at the moment. But please feel free to build on this repository to do it yourself (or my IsExternalInit repo, because it's less complex configuration-wise).