img2webp: add help note about arguments from a file

this was added in:
94a8377b extract the command-line parsing helpers to example_util

and matches the help in webpmux

https://groups.google.com/a/webmproject.org/d/msg/webp-discuss/DJs-w_-Id6o/svFXs2CqBgAJ

BUG=webp:101

Change-Id: I2944d1fb1ed3030c356960be2a6c8de15a79311f
(cherry picked from commit b6284d8247)

examples/img2webp.c

@@ -61,6 +61,10 @@ static void Help(void) {
   printf("\n");
   printf("example: img2webp -loop 2 in0.png -lossy in1.jpg\n"
          "                  -d 80 in2.tiff -o out.webp\n");
+  printf("\nNote: if a single file name is passed as the argument, the "
+         "arguments will be\n");
+  printf("tokenized from this file. The file name must not start with "
+         "the character '-'.\n");
 }
 
 //------------------------------------------------------------------------------