ffmpeg: document alternative to the lossless libx264 presets

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -4107,6 +4107,9 @@ static int opt_preset(OptionsContext *o, const char *opt, const char *arg)
                                            subtitle_codec_name;
 
     if (!(f = get_preset_file(filename, sizeof(filename), arg, *opt == 'f', codec_name))) {
+        if(!strncmp(arg, "libx264-lossless", strlen("libx264-lossless"))){
+            fprintf(stderr, "Please use -preset <speed> -qp 0\n");
+        }else
             fprintf(stderr, "File for preset '%s' not found\n", arg);
         exit_program(1);
     }