Add documentation for MR's fast-forward merge method

doc/user/project/merge_requests/fast_forward_merge.md

+# Fast-forward merge requests
+
+Retain a linear Git history and a way to accept merge requests without
+creating merge commits.
+
+## Overview
+
+When the fast-forward merge ([`--ff-only`][ffonly]) setting is enabled, no merge
+commits will be created and all merges are fast-forwarded, which means that
+merging is only allowed if the branch could be fast-forwarded.
+
+When a fast-forward merge is not possible, the user must rebase the branch manually.
+
+## Use cases
+
+Sometimes, a workflow policy might mandate a clean commit history without
+merge commits. In such cases, the fast-forward merge is the perfect candidate.
+
+## Enabling fast-forward merges
+
+1. Navigate to your project's **Settings** and search for the 'Merge method'
+1. Select the **Fast-forward merge** option
+1. Hit **Save changes** for the changes to take effect
+
+Now, when you visit the merge request page, you will be able to accept it
+**only if a fast-forward merge is possible**.
+
+![Fast forward merge request](img/ff_merge_mr.png)
+
+If the target branch is ahead of the source branch, you need to rebase the
+source branch before you will be able to do a fast-forward merge.
+
+![Fast forward merge rebase](img/ff_merge_rebase.png)
+
+[ffonly]: https://git-scm.com/docs/git-merge#git-merge---ff-only

doc/user/project/merge_requests/index.md

@@ -23,12 +23,13 @@ With GitLab merge requests, you can:
 - Organize your issues and merge requests consistently throughout the project with [labels](../../project/labels.md)
 - Add a time estimation and the time spent with that merge request with [Time Tracking](../../../workflow/time_tracking.html#time-tracking)
 - [Resolve merge conflicts from the UI](#resolve-conflicts)
+- Enable [fast-forward merge requests](https://docs.gitlab.com/ce/user/project/merge_requests/fast_forward_merge.html)
+
 
 With **[GitLab Enterprise Edition][ee]**, you can also:
 
 - View the deployment process across projects with [Multi-Project Pipeline Graphs](https://docs.gitlab.com/ee/ci/multi_project_pipeline_graphs.html#multi-project-pipeline-graphs) (available only in GitLab Enterprise Edition Premium)
 - Request [approvals](https://docs.gitlab.com/ee/user/project/merge_requests/merge_request_approvals.html) from your managers (available in GitLab Enterprise Edition Starter)
-- Enable [fast-forward merge requests](https://docs.gitlab.com/ee/user/project/merge_requests/fast_forward_merge.html) (available in GitLab Enterprise Edition Starter)
 - [Squash and merge](https://docs.gitlab.com/ee/user/project/merge_requests/squash_and_merge.html) for a cleaner commit history (available in GitLab Enterprise Edition Starter)
 - Enable [semi-linear history merge requests](https://docs.gitlab.com/ee/user/project/merge_requests/index.html#semi-linear-history-merge-requests) as another security layer to guarantee the pipeline is passing in the target branch (available in GitLab Enterprise Edition Starter)
 - Analise the impact of your changes with [Code Quality reports](https://docs.gitlab.com/ee/user/project/merge_requests/code_quality_diff.html) (available in GitLab Enterprise Edition Starter)
@@ -89,6 +90,15 @@ in a merged merge requests or a commit.
 
 [Learn more about cherry-picking changes.](cherry_pick_changes.md)
 
+## Fast-forward merge requests
+
+> Included in [GitLab Enterprise Edition Starter][products].
+
+If you prefer a linear Git history and a way to accept merge requests without
+creating merge commits, you can configure this on a per-project basis.
+
+[Read more about fast-forward merge requests.](fast_forward_merge.md)
+
 ## Merge when pipeline succeeds
 
 When reviewing a merge request that looks ready to merge but still has one or