Change changelog to use merge=union

This adjusts the .gitattributes file to allow git to understand how to
better merge the changelog using merge=union.

> If there are conflicts, the user should edit the result and delete one of the alternatives. When --ours, --theirs, or --union option is in effect, however, these conflicts are resolved favouring lines from <current-file>, lines from <other-file>, or lines from both respectively. The length of the conflict markers can be given with the --marker-size option.

See also:

1. https://about.gitlab.com/blog/2015/02/10/gitlab-reduced-merge-conflicts-by-90-percent-with-changelog-placeholders/
2. https://git-scm.com/docs/git-merge-file
3. https://about.gitlab.com/blog/2018/07/03/solving-gitlabs-changelog-conflict-crisis/

No idea if this will work on github dot com, but it may work on git
locally.

.gitattributes

@@ -3,3 +3,4 @@
 *.sln text eol=crlf
 *.csproj text eol=crlf
 *.vsmdi text eol=crlf
+CHANGELOG.md merge=union