Add Avo engine routes path to the regular `routes.rb` file on install

In order to nto have conflicts in how some link helpers work, we can add this snippet to the parent app on install so users know to use it.

```ruby


if defined? ::Avo
  Avo::Engine.routes.draw do
    scope :resources do
      get "courses/cities", to: "courses#cities"
      get "users/get_users", to: "users#get_users"
    end
  end
end
```