add rule to keep disscussion on the public mailinglist (stolen from mplayers policy)

(like always 1 complain from a ffmpeg-developer and ill reverse and we can disscuss it)

Originally committed as revision 8413 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/ffmpeg-doc.texi

@@ -1551,6 +1551,9 @@ please use av_log() instead.
     Update the documentation if you change behavior or add features. If you are
     unsure how best to do this, send a patch to ffmpeg-devel, the documentation
     maintainer(s) will review and commit your stuff.
+@item
+    Try to keep important discussions and requests (also) on the public
+    developer mailing list, so that all developers can benefit from them.
 @item
     Never write to unallocated memory, never write over the end of arrays,
     always check values read from some untrusted source before using them