Update README.md

sdk/cosmos/azure-cosmos/README.md

@@ -488,7 +488,7 @@ operation_headers = operation_response.get_response_headers()
 etag_value = operation_headers['etag']
 request_charge = operation_headers['x-ms-request-charge']
 ```
-For queries, response headers will be present when using paging only:
+For queries, response headers will be present when using paging only - and will only be populated once you start going through your pages:
 ```python
 from azure.cosmos import CosmosClient
 import os
@@ -503,6 +503,7 @@ container = database.get_container_client(CONTAINER_NAME)
 
 query_response = container.query_items("select * from c", enable_cross_partition_query=True)
 query_pages = query_response.by_page()
+empty_headers = query_pages.get_response_headers()
 first_page = query_pages.next()
 first_page_headers = query_pages.get_response_headers()
 etag_value = first_page_headers['etag']