Clarify codec addition instructions.

Originally committed as revision 4853 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/ffmpeg-doc.texi

@@ -1278,8 +1278,11 @@ please use av_log() instead.
     Incrementing the third component means a noteworthy binary compatible
     change (e.g. encoder bug fix that matters for the decoder).
 @item
-    If you add a new codec, remember to update the changelog, add it to the
-    supported codecs table and bump the version number appropriately.
+    If you add a new codec, remember to update the changelog, add it to
+    the supported codecs table in the documentation and bump the second
+    component of the @file{libavcodec} version number appropriately. If
+    it has a fourcc, add it to @file{libavformat/avienc.c}, even if it
+    is only a decoder.
 @end enumerate
 
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