INSTALL.Cygwin: Document that Cygwin has no new enough Qt
Cygwin has only Qt5-5.9. Freeciv-3.1 and later require at least Qt5-5.11. For years I've waited the situation in Cygwin to improve, but nothing has happened. It's better to document the situation in INSTALL.Cygwin than to wait with it further. Currently the document claims that Qt-client can be built in Cygwin (like Qt-client of earlier branches can).