add note about extensions

Author: yaacovCR

spec/Section 6 -- Execution.md

@@ -366,6 +366,14 @@ Note: The Incremental Publisher may be run as a separate service. The input to
 the Incremental Publisher are the Execution Events enqueued on {eventQueue}; the
 output is the value for {pending}, as well as the {subsequentResults} stream.
 
+Note: The Incremental Publisher could be extended to provide additional
+functionality. For example, for implementations intending to defer execution
+until the parent result has completed, the Incremental Publisher could be
+extended to allow subscribing to this event. Additionally, for implementations
+choosing to ignore incremental delivery directives given a certain threshold of
+already deferred fields, the Incremental Publisher could be extended to allow
+querying for the present number of deferred fields.
+
 #### Incremental Result Records
 
 An Incremental Result Record is either an Initial Result Record or a Subsequent