doc/APIchanges: Add 4.0 cut point

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/APIchanges

@@ -15,6 +15,8 @@ libavutil:     2017-10-21
 
 API changes, most recent first:
 
+-------- 8< --------- FFmpeg 4.0 was cut here -------- 8< ---------
+
 2018-04-03 - d6fc031caf - lavu 56.13.100 - pixdesc.h
   Deprecate AV_PIX_FMT_FLAG_PSEUDOPAL and make allocating a pseudo palette
   optional for API users (see AV_PIX_FMT_FLAG_PSEUDOPAL doxygen for details).