[css-break-4] Add link to issue

Author: fantasai

css-break-4/Overview.bs

@@ -103,6 +103,7 @@ Adjoining Margins at Breaks: the 'margin-break' property</h3>
 	it is otherwise discarded.
 	Do we want to include this feature?
 	Also, if only one ''keep'' is specified, does it apply only to the before margin or both sides?
+	See <a href="https://github.com/w3c/csswg-drafts/issues/3254">discussion</a>.
 
 	<div class="example">
 		Consider the following <a>multi-column layout</a> example: