Merge branch 'ab-changelog-docs' into 'master'

Performance improvements warrants a changelog entry.

See merge request gitlab-org/gitlab-ce!18218

doc/development/changelog.md

@@ -44,6 +44,7 @@ the `author` field. GitLab team members **should not**.
 - _Any_ contribution from a community member, no matter how small, **may** have
   a changelog entry regardless of these guidelines if the contributor wants one.
   Example: "Fixed a typo on the search results page. (Jane Smith)"
+- Performance improvements **should** have a changelog entry.
 
 ## Writing good changelog entries