add documentation file

doc/workflow/rebase_before_merge.md

+#Rebase before merge
+
+GitLab Enterprise Edition offers a way to rebase before merging a merge request. You can configure this per project basis by navigating to the project settings page and selecting `Merge Requests Rebase` checkbox.
+
+![Merge request settings](merge_request_settings.png)
+
+Before accepting a merge request, select `rebase before merge`.
+![Merge request widget](merge_request_widget.png)
+
+GitLab will attempt to cleanly rebase before merging branches. If clean rebase is not possible, regular merge will be performed.
+If clean rebase is possible and history of the traget branch will be altered with the the merge.
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