Add APIchanges entry after avfilter_open() change in r24765.

Originally committed as revision 24766 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/APIchanges

@@ -13,6 +13,13 @@ libavutil:   2009-03-08
 
 API changes, most recent first:
 
+2010-08-11 - r24765 - lavfi 1.33.0 - avfilter_open()
+  Change avfilter_open() signature:
+  AVFilterContext *avfilter_open(AVFilter *filter, const char *inst_name) ->
+  int avfilter_open(AVFilterContext **filter_ctx, AVFilter *filter, const char *inst_name);
+
+  This change breaks libavfilter API/ABI.
+
 2010-08-11 - r24763 - lavfi 1.32.0 - AVFilterBufferRef
   Add a type field to AVFilterBufferRef, and move video specific
   properties to AVFilterBufferRefVideoProps.