Add Some Extra Documentation Around Mass Insert

Including how to add a user as a maintainer to any number of the newly
inserted projects.

doc/development/README.md

@@ -69,6 +69,7 @@ description: 'Learn how to contribute to GitLab.'
 - [Developing against interacting components or features](interacting_components.md)
 - [File uploads](uploads.md)
 - [Auto DevOps development guide](auto_devops.md)
+- [Mass Inserting Models](mass_insert.md)
 - [Cycle Analytics development guide](cycle_analytics.md)
 
 ## Performance guides

doc/development/mass_insert.md

+# Mass Inserting Rails Models
+
+Setting the environment variable [`MASS_INSERT=1`](rake_tasks.md#env-variables)
+when running `rake setup` will create millions of records, but these records
+aren't visible to the `root` user by default.
+
+To make any number of the mass-inserted projects visible to the `root` user, run
+the following snippet in the rails console.
+
+```ruby
+u = User.find(1)
+Project.last(100).each { |p| p.set_create_timestamps && p.add_maintainer(u, current_user: u) } # Change 100 to whatever number of projects you need access to
+```