Add merge=union for the CHANGELOG file (#339)

This should reduce the amount of PR conflict

Author: patch0

.gitattributes

@@ -0,0 +1 @@
+CHANGELOG.md merge=union

CHANGELOG.md

@@ -22,6 +22,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Change port number to fix 'no consent token' error (#326)
 - Long file names truncated rather than scrolling in left hand file pane (#316, #337)
 - Tab scrollbars only appear when necessary (#331)
+- Added merge=union gitattribute for this file (#339)
 
 ### Fixed
 - Make sure button text is always centered (#328)