http://go.microsoft.com/fwlink/p/?LinkId=389848
Universal Windows app samples:
http://code.msdn.microsoft.com/windowsapps/Universal-Windows-app-cb3248c3
Build apps that target Windows and Windows Phone 8.1 by using Visual Studio (Windows Runtime apps using C#/VB/C++ and XAML):
http://msdn.microsoft.com/en-us/library/windows/apps/dn609832.aspx#addsupport
Dev Center now open for Windows Phone 8.1 and universal Windows app submissions:
http://blogs.windows.com/windows/b/buildingapps/archive/2014/04/14/dev-center-now-open-for-windows-phone-8-1-and-universal-windows-app-submissions.aspx