Add a note about what to do when adding a new codec.

Originally committed as revision 4848 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/ffmpeg-doc.texi

@@ -1277,6 +1277,9 @@ please use av_log() instead.
     (e.g. addition of a function).
     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.
 @end enumerate
 
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