Update DS requirements

Author: jazzz

standards/application/chat-framework.md

@@ -46,8 +46,10 @@ A Delivery Service (DS) is the service or method that distributes payloads to cl
 
 - A DS MUST provide a method for clients to subscribe to messages from a delivery_address
 - Payloads sent to a delivery_address are delivered by a DS to all subscribers of that delivery_address
+- A DS SHOULD handle segmentation if the underling transport limits message sizes
 - A DS MAY NOT guarantee message delivery
 - A DS MAY NOT guarantee message ordering
+- A DS MAY reject payloads 
 
 
 ### Framing Strategy