Add .NETStandard 1.6

[ericstj]

We need a .NETStandard1.6. It should be mapped as follows:

• net4.6.3 > NETStandard1.6
• netcoreapp1.0 > NETStandard1.6

/cc @joelverhagen
/cc @richlander @Petermarcu @weshaggard @joshfree

[joelverhagen]

Confirmations:

• This changes netcoreapp1.0 from netstandard1.5 to netstandard1.6.
• dotnet5.7 will not have any explicit mappings.
• netstandardapp1.6 will not have any explicit mappings.

[joelverhagen]

Also:

• All Xamarin TFMs should move up to netstandard1.6 (according to the previous decisions that Xamarin should track highest .NETStandard version).

[ericstj]

This changes netcoreapp1.0 from netstandard1.5 to netstandard1.6.

Correct

dotnet5.7 will not have any explicit mappings.

Correct

netstandardapp1.6 will not have any explicit mappings.

Folks have been discussion what we want to do with this TFM now that we have netcoreapp1.0. I'll let @richlander chime in if he wants this.

All Xamarin TFMs should move up to netstandard1.6 (according to the previous decisions that Xamarin should track highest .NETStandard version).

Correct

[joelverhagen]

Closed with NuGet/NuGet.Client#511.