Add developmenty documentation about routing

Explain and recommend developers to use `/-/` scope for group and
project routes in Rails
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

doc/development/README.md

@@ -59,6 +59,7 @@ description: 'Learn how to contribute to GitLab.'
 - [DeclarativePolicy framework](policies.md)
 - [How Git object deduplication works in GitLab](git_object_deduplication.md)
 - [Geo development](geo.md)
+- [Routing](routing.md)
 
 ## Performance guides
 

doc/development/routing.md

+# Routing
+
+The GitLab backend is written primarily with Rails so it uses [Rails
+routing](https://guides.rubyonrails.org/routing.html). Beside Rails best
+practices, there are few rules unique to the GitLab application. To
+support subgroups, GitLab project and group routes use the wildcard
+character to match project and group routes. For example, we might have
+a path such as:
+
+    /gitlab-com/customer-success/north-america/west/customerA
+
+However, paths can be ambiguous. Consider the following example:
+
+    /gitlab-com/edit
+
+It's ambiguous whether there is a subgroup named `edit` or whether
+this is a special endpoint to edit the `gitlab-com` group.
+
+To eliminate the ambiguity and to make the backend easier to maintain,
+we introduced the `/-/` scope. The purpose of it is to separate group or
+project paths from the rest of the routes. Also it helps to reduce the
+number of [reserved names](../user/reserved_names.md).
+
+## Global routes
+
+We have a number of global routes. For example:
+
+    /-/health
+    /-/metrics
+
+## Group routes
+
+Every group route must be under the `/-/` scope.
+
+Examples:
+
+    gitlab-org/-/edit
+    gitlab-org/-/activity
+    gitlab-org/-/security/dashboard
+    gitlab-org/serverless/-/activity
+
+To achieve that, use the `scope '-'` method.
+
+## Project routes
+
+Every project route must be under the `/-/` scope, except cases where a Git
+client or other software requires something different.
+
+Examples:
+
+    gitlab-org/gitlab-ce/-/activity
+    gitlab-org/gitlab-ce/-/jobs/123
+    gitlab-org/gitlab-ce/-/settings/repository
+    gitlab-org/serverless/runtimes/-/settings/repository
+
+Currently, only some project routes are placed under the `/-/` scope. However,
+you can help us migrate more of them! To migrate project routes:
+
+1. Modify existing routes by adding `-` scope.
+1. Add redirects for legacy routes by using `Gitlab::Routing.redirect_legacy_paths`.
+1. Create a technical debt issue to remove deprecated routes in later releases.
+
+To get started, see an [example merge request](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28435).