WinRT: added details of the port's status, to README-winrt.md
WinRT: made README-winrt.md slightly easier to read in an 80-column text editor
WinRT: moved documentation + setup related TODO items out of README-winrt.md

The TODO items were moved to SDL Bugzilla, case 2775 (https://bugzilla.libsdl.org/show_bug.cgi?id=2775).