Added a README file regarding WinRT support

To note, this file is currently formatted with CRLF line endings, rather than
LF, to allow the file to be viewed with Notepad.

README-winrt.txt

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+================================================================================
+Simple DirectMedia Layer for WinRT
+================================================================================
+
+SDL/WinRT layer allows SDL2-based applications to run on many of Microsoft's
+platforms that utilize the "Windows Runtime" (aka "WinRT") APIs.  WinRT apps
+are currently always full-screen apps, run in what Microsoft calls their
+"Modern" environment (aka. "Metro"), and are distributed via Microsoft-run
+online stores.  Some of the operating systems that support such apps include:
+
+* Windows 8.x
+* Windows RT 8.x (aka. Windows 8.x for ARM processors)
+* Windows Phone 8.x
+
+To note, WinRT applications that run on Windows 8.x and/or Windows RT are often
+called "Windows Store" apps.
+
+
+--------------------------------------------------------------------------------
+Requirements
+--------------------------------------------------------------------------------
+- Microsoft Visual C++ 2012 -- Free, "Express" editions may be used, so long
+  as they include support for either "Windows Store" or "Windows Phone" apps.
+  (NOTE: MSVC 2013 support is pending.  2012 projects may be converted to 2013
+  projects by MSVC, in the meantime.)
+- A valid Microsoft account -- This requirement is not imposed by SDL, but
+  rather by Microsoft's Visual C++ toolchain.
+
+
+--------------------------------------------------------------------------------
+TODO
+--------------------------------------------------------------------------------
+- Finish adding support for MSVC 2013, and "Universal" WinRT apps, which
+  support Windows 8.1, Windows Phone 8.1, and in the future, Xbox One and
+  Windows Desktop.
+- Finish adding support for the SDL satellite libraries (SDL_image, SDL_mixer,
+  SDL_ttf, etc.)
+- Create templates for both MSVC 2012 and MSVC 2013, and have the corresponding
+  VSIX packages either include pre-built copies of SDL, or reference binaries
+  available via MSVC's NuGet servers
+- Write setup instructions that use MSVC 201x templates
+- Write setup instructions that don't use MSVC 201x templates, and use
+  MSVC project-to-project references, rather than pre-built binaries
+- Write a list of caveats found in SDL/WinRT, such as APIs that don't work due
+  to platform restrictions, or things that need further work