README: Update Windows build requirements

Change-Id: I9e29d0728db35f65822aff9d709cd93a6b3b7ab2
Task-number: QTBUG-54084
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

README

@@ -39,8 +39,10 @@ HOW TO BUILD QT5
    Windows:
    --------
 
-     Open a Windows SDK (7.0, 7.1 or later) command prompt. Ensure that the
-     following tools can be found in the path:
+     Open a command prompt.
+     Ensure that the following tools can be found in the path:
+     * Supported compiler (Visual Studio 2012 or later,
+        MinGW-builds gcc 4.9 or later)
      * Perl version 5.12 or later   [http://www.activestate.com/activeperl/]
      * Python version 2.7 or later  [http://www.activestate.com/activepython/]
      * Ruby version 1.9.3 or later  [http://rubyinstaller.org/]