Migration Guidance for .NET Core 1.x to .NET Core 2.0

[BillWagner]

Developers that have invested in .NET Core 1.x will need guidance for what they must do to update their libraries and applications to .NET Core 2.0.

This topic will explain those steps, following the pattern for .NET Framework migrations.

[dasMulli]

Should likely include the breaking change made by introducing implicit AssetTargetFallback values in .NET Core 2.0 / .NET Standard 2.0. See #2816 (comment)