Add support for toplevel JSON API links.

lib/active_model/serializable_resource.rb

@@ -1,7 +1,7 @@
 require 'set'
 module ActiveModel
   class SerializableResource
-    ADAPTER_OPTION_KEYS = Set.new([:include, :fields, :adapter, :meta, :meta_key])
+    ADAPTER_OPTION_KEYS = Set.new([:include, :fields, :adapter, :meta, :meta_key, :links])
 
     # Primary interface to composing a resource with a serializer and adapter.
     # @return the serializable_resource, ready for #as_json/#to_json/#serializable_hash.

lib/active_model/serializer/adapter/json_api.rb

@@ -65,6 +65,11 @@ module ActiveModel
 
           ApiObjects::JsonApi.add!(hash)
 
+          if instance_options[:links]
+            hash[:links] ||= {}
+            hash[:links].update(instance_options[:links])
+          end
+
           hash
         end