Update gitattributes to use the union merge driver for release notes (#11252)

Author: fabiocav

.gitattributes

@@ -48,4 +48,8 @@
 *.vbproj text=auto
 *.fsproj text=auto
 *.dbproj text=auto
-*.sln text=auto eol=crlf
+*.sln text=auto eol=crlf
+
+# Use union on release notes to keep both local and remote changes
+# and avoid merge conflicts.
+release_notes.md merge=union