doc/community: update the rules according to voting results

Cf.:
* http://lists.ffmpeg.org/pipermail/ffmpeg-devel/2023-October/316054.html
* http://lists.ffmpeg.org/pipermail/ffmpeg-devel/2023-November/316618.html

doc/community.texi

@@ -28,6 +28,8 @@ The General Assembly is made up of active contributors.
 
 Contributors are considered "active contributors" if they have pushed more than 20 patches in the last 36 months in the main FFmpeg repository, or if they have been voted in by the GA.
 
+The list of active contributors is updated twice each year, on 1st January and 1st July, 0:00 UTC.
+
 Additional members are added to the General Assembly through a vote after proposal by a member of the General Assembly. They are part of the GA for two years, after which they need a confirmation by the GA.
 
 A script to generate the current members of the general assembly (minus members voted in) can be found in `tools/general_assembly.pl`.