Don't use `#to_s` to display models in Administrate.

Created by: rikkipitt

Just a minor note to request the documentation of the use of the to_s method for better representing objects across the various Administrate dashboards. E.g. has_many select lists and belongs_to on index lists. This might catch out some newer Rails/Ruby users if not explicitly mentioned...

Something along the lines of:

to_s is the standard Ruby method for converting an object to a string. You can define to_s on your model class when you want a custom string representation.

Example:

class Tag < ActiveRecord::Base
  def to_s
    name
  end
end

Thus instead of displaying #<Tag:0x007fad07b7ad98>, we can display the actual name of the tag.

Thoughts?