Add routing guide doc on root with namespaces.
I read #9419 and realized I didn't know you could have namespaced roots, checked around, and a few friends didn't either. Figured this makes it guide worthy.
|@@ -797,6 +797,16 @@ You should put the `root` route at the top of the file, because it is the most p|
|NOTE: The `root` route only routes `GET` requests to the action.|
|+You can also use root inside namespaces and scopes as well. For example:|
|+namespace :admin do|
|+ root to: "admin#index"|
|+root to: "home#index"|
|### Unicode character routes|
|You can specify unicode character routes directly. For example:|