Change merge driver for CHANGELOG.md to resolve conflict problem

Currently Problem
------

CHANGELOG.md is very often conflict when PR.
It's very troublesome.
e.g. rubocop#3588 (comment)

Solution
-----

I've added a `.gitattributes` file.
The file changes a merge driver to union for CHANGELOG.md.

The union driver has what it takes to merge CHANGELOG.md.
With the setting, CHANGELOG doesn't conflict!
Always both of the addition is chosen.

However, this solution has a problem.
GitHub doesn't support the feature.
So, We can't merge a PR at GitHub Web if CHANGELOG.md conflicts.

Don't worry, `hub` command supports the feature.
Maintainer can merge a conflicted PR by hub command.
For example.

```sh
$ git branch
* master
some-feature-branch-1

$ hub merge https://github.com/bbatsov/rubocop/pull/xxxx # specify a PR url
$ git push
```

I think this method is not complete, but it is better than now.
What do you think?

More Information
-----

- http://krlmlr.github.io/using-gitattributes-to-avoid-merge-conflicts/

.gitattributes

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