Clarify .NET Core support in nupkg summary

[chrisdunelm]

Summary currently shows support for netstandard1.3, but doesn't explicitly mention that this provides full .NET Core support. This can be confusing.

[LindaLawton]

When adding it make sure to note this does not include #933 or we will have even more confusion.

Update the README as well please. Might as well be consistent. Any way we can have the nupkg summary build using the text from the README?