Documentation for approval feature

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

doc/workflow/README.md

@@ -4,6 +4,7 @@
 - [Project forking workflow](forking_workflow.md)
 - [Project Features](project_features.md)
 - [Authorization for merge requests](authorization_for_merge_requests.md)
+- [Approval for merge requests](approval_for_merge_requests.md)
 - [Rebase before merge](rebase_before_merge.md)
 - [Groups](groups.md)
 - [Share projects with other groups](share_projects_with_other_groups.md)

doc/workflow/approval_for_merge_requests.md

+# Approval for merge requests
+
+Usually you accept merge request if it is ready and reviewed. But in certain 
+cases you want to be sure that every merge request will be reviewed by several 
+people before it gets merged. You can use GitLab approve feature for enforcing such 
+workflow. To enable it you should go to project settings page and set 
+"Approvals required" field to numeric value. For example if you set it to number 3 
+that means each merge request should receive 3 approves from different people 
+before it can be merged via user interface.
+
+
+![approve_setting](approval_for_merge_requests/settings.png)
+
+After you set approves value you can see on merge request page "Approve" button
+instead of "Accept" one. As soon as merge request get enough approvals it will 
+be available for merge with user interface.
+
+![approve_merge_request](approval_for_merge_requests/mr.png)