Add documenation for code quality diff feature

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

doc/user/project/merge_requests/code_quality_diff.md

+# Code quality diff
+
+> [Introduced][ee-1984] in GitLab Enterprise Edition 9.3.
+
+If you are using GitLab CI you can analyze your source code quality using
+Code Climate analyzer docker image. Then GitLab will show you how merge request
+changes source code quality right inside merge request widget.
+
+![Code Quality Widget][quality-widget]
+
+## Enabling feature
+
+Feature can be enabled by following next steps: 
+
+1. [Analyze project code quality with Code Climate CLI ][cc-docs] 
+2. Start a new merge request from branch that already has codeclimate job succeed
+3. Wait till merge request codeclimate job succeed
+  
+[ee-1984]: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1984
+[quality-widget]: img/code_quality.gif
+[cc-docs]: https://docs.gitlab.com/ce/ci/examples/code_climate.html#analyze-project-code-quality-with-code-climate-cli 

doc/user/project/merge_requests/index.md

@@ -105,6 +105,16 @@ creating merge commits, you can configure this on a per-project basis.
 
 [Read more about fast-forward merge requests.](fast_forward_merge.md)
 
+## Code quality diff
+
+> Included in [GitLab Enterprise Edition Starter][products].
+
+If you are using GitLab CI you can analyze your source code quality using
+Code Climate analyzer docker image. Then GitLab will show you how merge request
+changes source code quality right inside merge request widget.
+
+[Read more about code quality diff.](code_quality_diff.md)
+
 ## Ignore whitespace changes in Merge Request diff view
 
 If you click the **Hide whitespace changes** button, you can see the diff