doc/muxers/image2: document strftime option

Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

doc/muxers.texi

@@ -314,6 +314,17 @@ Note also that the pattern must not necessarily contain "%d" or
 ffmpeg -i in.avi -f image2 -frames:v 1 img.jpeg
 @end example
 
+The @option{strftime} option allows you to expand the filename with
+date and time information. Check the documentation of
+the @code{strftime()} function for the syntax.
+
+For example to generate image files from the @code{strftime()}
+"%Y-%m-%d_%H-%M-%S" pattern, the following @command{ffmpeg} command
+can be used:
+@example
+ffmpeg -f v4l2 -r 1 -i /dev/video0 -f image2 -strftime 1 "%Y-%m-%d_%H-%M-%S.jpg"
+@end example
+
 @table @option
 @item start_number @var{number}
 Start the sequence from @var{number}. Default value is 1. Must be a
@@ -324,6 +335,9 @@ If @var{number} is nonzero, the filename will always be interpreted as just a
 filename, not a pattern, and this file will be continuously overwritten with new
 images.
 
+@item strftime @var{[0|1]}
+If set to 1, expand the filename with date and time information from
+@code{strftime()}. Default value is 0.
 @end table
 
 The image muxer supports the .Y.U.V image file format. This format is