Adds net461 Target

[Silvenga]

Per best practices, when targeting netstandard2.0, a net461 should be included (https://docs.microsoft.com/en-us/dotnet/standard/library-guidance/cross-platform-targeting).

Functionally, this prevents needing to reference all of netstandard2.0 in .NET Framework (and prevents bugs in older version of .NET Framework). With modern .NET tooling, this should continue to build fine under Linux and friends.

[joaoportela]

The specific recommendation you mentioned:

[Silvenga]

Thanks!