build #993

I've moved to Xamarin.Android build #993 and Mono 5.14 and nuget,
including the new VSCode 1.25.1 and C# plugin and Monodevelop 7.5

Also I am using the dotnet sdk - it is necessary to build the netstandard2.0 projects and restore the nuget packages.

And I've updated the documentation:
https://goo.gl/mQvTJk

P.S. Monodevelop 7.5 doesn't open the Droid projects.