Visual Studio team just released Visual Studio 2017 v15.4 that now includes the updated UWP tooling to support .NET Standard 2.0. Earlier this month, Microsoft launched .NET Core 2.0 and .NET Standard 2.0. Check out What is new in .NET Core 2.0 to learn more about .NET Core 2.0 features.
The support of .NET Standard 2.0 in UWP is available on Windows Fall Creators Update (FCU) only. If you do not have FCU, you need to download and install it before you can build .NET Standard 2.0 UWP apps.
Once your Windows is updated, you need to download Visual Studio 2017 v15.4 Preview: https://www.visualstudio.com/vs/preview/
The last thing you will need is Windows Insider Preview SDK that installs all necessary tools for UWP.