Add documentation for the 'only_allow_merge_if_build_succeeds.rb' feature

Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/workflow/merge_requests.md

@@ -2,6 +2,17 @@
 
 Merge requests allow you to exchange changes you made to source code
 
+## Only allow merge requests to be merged if the build succeeds
+
+You can prevent merge requests from being merged if their build did not succeed
+in the project settings page.
+
+![only_allow_merge_if_build_succeeds](merge_requests/only_allow_merge_if_build_succeeds.png)
+
+Navigate to project settings page and select the `Only allow merge requests to be merged if the build succeeds` check box.
+
+Please note that you need to have builds configured to enable this feature.
+
 ## Checkout merge requests locally
 
 Locate the section for your GitLab remote in the `.git/config` file. It looks like this: