misc: 1.4.0 changelog

Author: kamilogorek

CHANGELOG.md

@@ -22,8 +22,13 @@ A major release `N` implies the previous release `N-1` will no longer receive up
 
 ## Unreleased
 
+- TBA
+
+# 1.4.0
+
 - No longer set the last event id for transactions #1186
 - Added support for client reports (disabled by default for now) #1181
+- Added `tracestate` header handling #1179
 
 ## 1.3.1