Add an entry for the avfilter_add_colorspace() API addition.

Originally committed as revision 21008 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/APIchanges

@@ -12,6 +12,9 @@ libavutil:   2009-03-08
 
 API changes, most recent first:
 
+2010-01-03 - r21007 - lavfi 1.13.0 - avfilter_add_colorspace()
+  Add avfilter_add_colorspace().
+
 2010-01-02 - r20998 - lavf 52.46.0 - av_match_ext()
   Add av_match_ext(), it should be used in place of match_ext().