Add an entry for the av_guess_format() addition.

Originally committed as revision 20992 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/APIchanges

@@ -12,6 +12,9 @@ libavutil:   2009-03-08
 
 API changes, most recent first:
 
+2010-01-01 - r20991 - lavf 52.45.0 - av_guess_format()
+  Add av_guess_format(), it should be used in place of guess_format().
+
 2009-12-13 - r20834 - lavf 52.43.0 - metadata API
   Add av_metadata_set2(), AV_METADATA_DONT_STRDUP_KEY and AV_METADATA_DONT_STRDUP_VAL.