doc: add windres to mingw *nix build command
# This Makefile is made for MinGW
#
# To cross compile on a *nix machine :
-# make -f Makefile.mingw DLLWRAP=mingw32-dllwrap CC=mingw32-gcc AR=mingw32-ar
+# make -f Makefile.mingw DLLWRAP=mingw32-dllwrap CC=mingw32-gcc AR=mingw32-ar WINDRES=mingw32-windres
#
SRCDIR = src
INCDIR = include
