Add APIchanges entry after libavcore addition.

Originally committed as revision 24407 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/APIchanges

@@ -2,6 +2,7 @@ Never assume the API of libav* to be stable unless at least 1 week has passed si
 the last major version increase.
 The last version increases were:
 libavcodec:  ?
+libavcore:   2010-07-21
 libavdevice: ?
 libavfilter: 2009-10-18
 libavformat: ?
@@ -12,6 +13,9 @@ libavutil:   2009-03-08
 
 API changes, most recent first:
 
+2010-07-21 - r24393 - lavcore 0.0.0
+  Add libavcore.
+
 2010-07-17 - r24291 - lavfi 1.25.0 - AVFilterBuffer
   Remove w and h fields from AVFilterBuffer.