doc/APIChanges: mention nb_threads addition to AVFilterContext

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

doc/APIchanges

@@ -15,6 +15,9 @@ libavutil:     2015-08-28
 
 API changes, most recent first:
 
+2016-08-29 - 4493390 - lavfi 6.58.100 - avfilter.h
+  Add AVFilterContext.nb_threads.
+
 2016-08-15 - c3c4c72 - lavc 57.53.100 - avcodec.h
   Add trailing_padding to AVCodecContext to match the corresponding
   field in AVCodecParameters.