WinRT: removed a completed TODO item from an inline comment
WinRT: added SDL_HINT_WINRT_PREF_PATH_ROOT

SDL_HINT_WINRT_PREF_PATH_ROOT allows WinRT apps to alter the path that
SDL_GetPrefPath() returns.  Setting it to "local" uses the app's
OS-defined Local folder, setting it to "roaming" uses the app's OS-defined
Roaming folder.

Roaming folder support is not available in Windows Phone 8.0.  Attempts to
make SDL_GetPrefPath() return a Roaming folder on this OS will be ignored.

Various bits of documentation on this were added to SDL_hints.h, and to
README-winrt.md