Update SerializableResource documentation (#1977)
Update SerializableResource documentation (#1977)

Mention that ActiveModelSerializers::SerializableResource.new can be used for
single resource as well as for collections
ScottKbka/active_model_serializers,Updater/active_model_serializers,nurey/active_model_serializers,groyoh/active_model_serializers,ScottKbka/active_model_serializers,nurey/active_model_serializers,groyoh/active_model_serializers,fabianoleittes/active_model_serializers,rails-api/active_model_serializers,Updater/active_model_serializers,groyoh/active_model_serializers,Updater/active_model_serializers,nurey/active_model_serializers,cassidycodes/active_model_serializers,ScottKbka/active_model_serializers