Document the fact that apps can declare their own root routes

Joint PR: bullet-train-co/bullet_train-core#1013

config/routes.rb

@@ -1,4 +1,5 @@
 Rails.application.routes.draw do
+
   # See `config/routes/*.rb` to customize these configurations.
   draw "concerns"
   draw "devise"
@@ -17,7 +18,9 @@
 
   scope module: "public" do
     # To keep things organized, we put non-authenticated controllers in the `Public::` namespace.
-    # The root `/` path is routed to `Public::HomeController#index` by default.
+    # The root `/` path is routed to `Public::HomeController#index` by default. You can set it
+    # to whatever you want by doing something like this:
+    # root to: "my_new_root_controller#index"
   end
 
   namespace :webhooks do
@@ -35,6 +38,10 @@
 
   namespace :account do
     shallow do
+      # The account root `/` path is routed to `Account::Dashboard#index` by default. You can set it
+      # to whatever you want by doing something like this:
+      # root to: "some_other_root_controller#index", as: "dashboard"
+
       # user-level onboarding tasks.
       namespace :onboarding do
         # routes for standard onboarding steps are configured in the `bullet_train` gem, but you can add more here.