Reword GIF section of README.1st

[<tim...@en...>]

Hi,

The notes about GIF output in README.1st are outdated. Attached is a=20
patch as per my understanding, but I would like to see it confirmed=20
before committing. Please tell me if it is ok.

As far as the ongoing process towards 4.2 is concerned, we still have to=20
update the copyrights years of all files, increment VERSION, and review=20
the couple of doc files (README, INSTALL, etc).

Best regards,

Timoth=C3=A9e