doc: document building for Windows RT.

doc/building.dox

@@ -361,6 +361,33 @@ contents in `toolchains/` subdirectory.
     git submodule init
     git submodule update
 
+@subsection building-cross-winrt Crosscompiling for Windows RT
+
+As said above, you need native build of `corrade-rc` executable. The below
+script assumes that native Corrade build is installed in `C:/Sys` and the
+installation path for WinRT dependencies is in `C:/Sys-winrt`.
+
+You need at least Windows 8.1, Visual Studio 2013 and Windows 8.1 Store/Phone
+SDK installed. Windows RT applications support OpenGL only through ANGLE, which
+is currently limited to OpenGL ES. Download and build ANGLE @ref building-windows-angle "according to instructions above",
+but use project files from the `winrt/` directory instead. Upcoming version
+2.0.4 of SDL has support for WinRT applications, download the source from
+https://www.libsdl.org/hg.php and use project files from the `VisualC-WinRT`
+directory. Because WinRT applications run in a sandbox, it's recommended to
+build the library as static so you don't have to bundle all the DLLs. Example:
+
+    mkdir build-winrt
+    cd build-winrt
+    cmake -DCORRADE_RC_EXECUTABLE="C:/Sys/bin/corrade-rc.exe" -DCMAKE_INSTALL_PREFIX="C:/Sys-winrt" -DBUILD_STATIC=ON -DWITH_SDL2APPLICATION=ON -DCMAKE_SYSTEM_NAME=WindowsStore -DCMAKE_SYSTEM_VERSION=8.1 -G "Visual Studio 14 2015" ..
+    cmake --build .
+
+Change `WindowsStore` to `WindowsPhone` if you want to build for Windows Phone
+instead. When done, you can install the package using `cmake --build . --target install`
+to make it available for depending projects.
+
+See @ref Platform::Sdl2Application documentation for more information about
+building your projects for WinRT.
+
 @subsection building-cross-win Crosscompiling for Windows using MinGW-w64
 
 @note This guide is tailored mainly for crosscompiling from ArchLinux. For