Adds warning about change of array root behaviour in 1.0
Adds warning about change of array root behaviour in 1.0

onomated/grape-active_model_serializers,fogisland/grape-active_model_serializers,kulte/grape-active_model_serializers,gaugau/grape-active_model_serializers,ruby-grape/grape-active_model_serializers,Thanx/grape-active_model_serializers,jrhe/grape-active_model_serializers