Clarify .NET Standard versions (2.0 or 2.1)

[goldfischli]

I Just tried to add the Nuget package and failed as it is for .NET Standard 2.1 and my Project .NET Framework 4.8.
I went back to 1.0.2, which is .NET Standard 2.0 and works with Framework.

The documentation only mentions .NET Standard 2.0.

Please consider updating the documentation to

• state the correct .NET Standard version (2.1)
• maybe add a hint for the .NET Framework guys to which version they could fall back to

thanks.