[css-grid-2] Add reference to gap issue.

Author: fantasai

css-grid-2/Overview.bs

@@ -204,6 +204,9 @@ Characteristics of a Subgrid Item</h3>
 			(Their items are instead spaced by the corresponding gutters of their parent grid,
 			since they are laid out in the parent grid.)
 
+			ISSUE: See <a href="https://github.com/w3c/csswg-drafts/issues/2285">issue 2285</a>
+			for discussion about customizing gaps within the subgrid.
+
 		<li>
 			The <a>subgrid</a> does not have any <a>implicit grid tracks</a>
 			in the subgridded dimension(s);