doc/muxers: document animated GIF muxer.

doc/muxers.texi

@@ -146,6 +146,40 @@ ffmpeg -i INPUT -f framemd5 -
 
 See also the @ref{md5} muxer.
 
+@anchor{gif}
+@section gif
+
+Animated GIF muxer.
+
+It accepts the following options:
+
+@table @option
+@item loop
+Set the number of times to loop the output. Use @code{-1} for no loop, @code{0}
+for looping indefinitely (default).
+
+@item final_delay
+Force the delay (expressed in centiseconds) after the last frame. Each frame
+ends with a delay until the next frame. The default is @code{-1}, which is a
+special value to tell the muxer to re-use the previous delay. In case of a
+loop, you might want to customize this value to mark a pause for instance.
+@end table
+
+For example, to encode a gif looping 10 times, with a 5 seconds delay between
+the loops:
+@example
+ffmpeg -i INPUT -loop 10 -final_delay 500 out.gif
+@end example
+
+Note 1: if you wish to extract the frames in separate GIF files, you need to
+force the @ref{image2} muxer:
+@example
+ffmpeg -i INPUT -c:v gif -f image2 "out%d.gif"
+@end example
+
+Note 2: the GIF format has a very small time base: the delay between two frames
+can not be smaller than one centi second.
+
 @anchor{hls}
 @section hls