img2webp: add help note about arguments from a file
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 b6284d8247b0bc2c27efa37cdbf3c6a46a0f1791)

PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp