README.txt should have link to WinRT Requirements

WinRT should be added to README.txt

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Additional Requirements for iOS, Android and WinRT
---------------------------------------------------

In order to develop iOS or Android applications, you need to install the
appropriate iOS or Android toolchains.

For information on how to install the Android toolchain, visit
http://qt-project.org/doc/qt-5/androidgs.html

For information on how to set up iOS projects, visit
http://qt-project.org/doc/qt-5/ios-support.html

For information on how to set up WinRT projects, visit
http://qt-project.org/doc/qt-5/winrt-support.html