Documentation of model using current_user from context hash

[danielpclark]

It is likely that people won't understand how to use the context hash as I didn't understand it at first. It would be good to have an example so I've written one for the README.

class PeopleResource < JSONAPI::Resource
  before_save do
    @model.user_id = context[:current_user].id if @model.new_record?
  end
end

[lgebhardt]

@danielpclark Thanks!