Add documentation for ff-merge and rebase

doc/workflow/README.md

@@ -15,6 +15,7 @@
 - [Project forking workflow](forking_workflow.md)
 - [Project users](add-user/add-user.md)
 - [Protected branches](protected_branches.md)
+- [Fast-forward merge](ff_merge.md)
 - [Rebase before merge](rebase_before_merge.md)
 - [Sharing a project with a group](share_with_group.md)
 - [Share projects with other groups](share_projects_with_other_groups.md)

doc/workflow/ff_merge.md

+# Fast-forward merge
+
+GitLab Enterprise Edition offers a way to accept merge request without creating merge commit.
+If you prefer linear git history - this might be a good feature for you.
+You can configure this per project basis by navigating to the project settings page and selecting `Only fast-forward merging` checkbox.
+
+![Merge request settings](ff_merge.png)
+
+Now when you visit merge request page you will be able to accept it only if fast-forward merge is possible. 
+If target branch is ahead of source branch - you need to rebase source branch before you will be able to do fast-forward merge.
+
+For simple rebase operations you can use [Rebase before merge](rebase_before_merge.md) feature. 

doc/workflow/rebase_before_merge.md

-#Rebase before merge
+# 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.
+GitLab Enterprise Edition offers a way to rebase source branch of merge request. 
+This feature is part of [Fast-forward merge](ff_merge.md) feature. 
+It allows you to rebase source branch of merge request in order to perform fast-forward merge.
+
+You can configure this per project basis by navigating to the project settings page and selecting `Rebase button` checkbox.
+This checkbox is visible only if you have `Only fast-forward merging` checkbox enabled.
 
 ![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 target branch will be altered with the merge.
+Now if fast-forward merge requires rebase - you will see rebase button:
+
+![Rebase request widget](rebase_request_widget.png)
+
+GitLab will attempt to rebase source branch. If rebase succeed you will see `Accept merge request` button.
+If clean rebase is not possible - you need to do rebase manually. 
+Possibly rebase requires some conflicts to be resolved by human.