Add APIchanges entry for request_sample_fmt.

doc/APIchanges

@@ -13,6 +13,9 @@ libavutil:   2011-04-18
 
 API changes, most recent first:
 
+2011-xx-xx - xxxxxxx - lavc 53.4.0 - AVCodecContext.request_sample_fmt
+  Add request_sample_fmt field to AVCodecContext.
+
 2011-05-10 - 188dea1 - lavc 53.3.0 - avcodec.h
   Deprecate AVLPCType and the following fields in
   AVCodecContext: lpc_coeff_precision, prediction_order_method,