Add documentation.

doc/workflow/README.md

@@ -24,3 +24,4 @@
 - [Releases](releases.md)
 - [Merge Requests](merge_requests.md)
 - ["Work In Progress" Merge Requests](wip_merge_requests.md)
+- [Repository Mirroring](repository_mirroring.md)

doc/workflow/repository_mirroring.md

+# Repository Mirroring
+
+You can set up a project to automatically have its branches and tags updated from an upstream repository.
+This is useful when a repository you're interested in is located on a different server, and you want to be able to browse its content and its activity using the familiar GitLab interface.
+
+Mirror repositories are updated every hour, and all new branches, tags and commits will be visible in the project's activity feed. Users with at least developer access to the project can also force an immediate update.
+
+![Project page](repository_mirroring/project_page.png)
+
+Since the repository on GitLab functions as a mirror of the upstream repository, you are advised not to push commits directly to the repository on GitLab. Instead, any commits should be pushed to the upstream repository, and will end up in the GitLab repository automatically within an hour, or when a forced update is initiated.
+
+If you do manually update a branch in the GitLab repository, the branch will become diverged from upstream, and GitLab will no longer automatically update this branch to prevent any changes from being lost.
+
+![Diverged branch](repository_mirroring/diverged_branch.png)
+
+You can set up Repository Mirroring by navigating to Project Settings > Repository Mirroring.
+
+![Settings](repository_mirroring/settings.png)