RFC: Document Cygwin-to-MinGW cross compilation

cygwin and msys-target compilers are incapable of building libuv, but can build everything else - saves user time and frustration to check for them up front and error early. ref #8488 and #6234
would it be reasonable to just autodetect this from uname, and just assume that cygwin == cross-compile-with-mingw-for-cygwin-arch?