Document inclusion of belongs_to ActiveRecord associations

With `has_many` and `has_one` tightly mirroring ActiveRecord's naming conventions, I think it can be [confusing](http://stackoverflow.com/questions/13125214/active-model-serializers-belongs-to) that there isn't a `belongs_to` method.

It could also be reasonable to alias `belongs_to` to `has_one`, but I don't see the point in creating new baggage.