Root route required by sidebar

[pablobm]

Since thoughtbot/administrate#1376, Administrate requires a root route by default. Otherwise it raises the following:

ActionView::Template::Error (undefined local variable or method `root_url' for #<#<Class:0x000055d0d8b83f40>:0x000055d0d8b81380>):
     8: %>
     9: 
    10: <nav class="navigation" role="navigation">
    11:   <%= link_to "Back to app", root_url, class: "button button--alt" %> 
    12: 
    13:   <% Administrate::Namespace.new(namespace).resources.each do |resource| %>
    14:     <%= link_to(
  
administrate (0.13.0) app/views/administrate/application/_navigation.html.erb:11
...

This PR adds this route, so that the dummy app can run.

[sedubois]

Duplicate of #31 🙂

[pablobm]

Ooops! 😅