Update README.md usage instructions

[rossshannon]

The current instructions in the README:

RailsAdmin.config do |config|
  config.model User do
    edit do
      field :latitude, :map
    end
  end
end

result in all the other fields in the model being hidden in rails_admin. I have updated the usage instructions to be

RailsAdmin.config do |config|
  config.model User do
    configure :latitude, :map
    configure(:longitude) { visible false }
  end
end

which will not affect other fields on the model. It also hides the model’s original longitude field, as this is not needed, since the plugin adds the dynamic lat/lng fields below the map.