Add an APIchanges note regarding the new rtp hinting flag

Originally committed as revision 23180 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/APIchanges

@@ -12,6 +12,9 @@ libavutil:   2009-03-08
 
 API changes, most recent first:
 
+2010-05-18 - r23161 - lavf 52.63.0 - AVFMT_FLAG_RTP_HINT
+  Add AVFMT_FLAG_RTP_HINT as possible value for AVFormatContext.flags
+
 2010-05-09 - r23066 - lavfi 1.20.0 - AVFilterPicRef
   Add interlaced and top_field_first fields to AVFilterPicRef.