[css-grid-2] Add issue about subgrid MBP sizing.

Author: fantasai

css-grid-2/Overview.bs

@@ -197,6 +197,9 @@ Characteristics of a Subgrid Item</h3>
 			but acts as if it was completely empty for sizing purposes
 			in the subgridded dimension.
 
+			ISSUE: Correct handling of the subgrid’s margins/border/padding
+			is an open issue, see <a href="https://github.com/w3c/csswg-drafts/issues/2592">discussion</a>.
+
 		<li id="subgrid-item-contribution">
 			The <a>subgrid</a>'s own <a>grid items</a> participate
 			in the sizing of its parent grid in the subgridded dimension(s)