Add new Engagement resource and new method for creating new marketing event engagements

lib/shopify_api/resources/engagement.rb

@@ -0,0 +1,4 @@
+module ShopifyAPI
+  class Engagement < Base
+  end
+end

lib/shopify_api/resources/marketing_event.rb

@@ -1,4 +1,8 @@
 module ShopifyAPI
   class MarketingEvent < Base
+    def add_engagements(engagements)
+      engagements = {engagements: Array(engagements)}
+      post(:engagements, {}, engagements.to_json)
+    end
   end
 end