Move subrip/text API change info from Changelog to doc/APIchanges.

Note that a lavf bump was missing so I'm using 54.28.100 as a reference.

Changelog

@@ -58,8 +58,6 @@ version 1.0:
 - volume measurement filter
 - Ut Video encoder
 - Microsoft Screen 2 decoder
-- Matroska demuxer now identifies SRT subtitles as AV_CODEC_ID_SUBRIP
-  instead of AV_CODEC_ID_TEXT
 - smartblur filter ported from MPlayer
 - CPiA decoder
 - decimate filter ported from MPlayer

doc/APIchanges

@@ -31,6 +31,10 @@ API changes, most recent first:
   Can be used as a stripped-down replacement for strptime(), on
   systems which do not support it.
 
+2012-08-25 - 2626cc4 - lavf 54.28.100
+  Matroska demuxer now identifies SRT subtitles as AV_CODEC_ID_SUBRIP instead
+  of AV_CODEC_ID_TEXT.
+
 2012-08-13 - xxxxxxx - lavfi 3.8.100 - avfilter.h
   Add avfilter_get_class() function, and priv_class field to AVFilter
   struct.