Set merge=union git attribute for CHANGELOG.md

Currently all PRs add a line to CHANGELOG.md; this means they almost always run into merge conflicts since they are adding different lines at the exact same place in the file.

With `merge=union`, git will automatically resolve these merge conflicts by keeping all lines from both sides.

.gitattributes

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