Wait a few months. Then target your library development at .net standard 2.0. It will work across the classic .net framework (Windows), the new cross-platform .net core (Windows, Linux, OSX) as well as the Xamarin (iOS, Android, Windows UWP).
That's exciting. I applaud MS for openly expanding to other platforms but the confusing profile names need to be simplified. IMHO, there should be two profiles .NET Universal and .NET Windows (a superset of .NET Universal).
something being a superset of a "universal" lib is not confusing?