Add description

Based on issue [#1753](https://github.com/sferik/rails_admin/issues/1573)
An option to have a description at the top of the list view of a model.

```ruby
RailsAdmin.config do |config|
   config.model Team do
     desc 'Description goes here'
     list do
       field :name
       field :price
     end
   end
end
```