[css-lists-3] Add link to open issue from issue description.

Author: fantasai

css-lists-3/Overview.bs

@@ -1263,6 +1263,8 @@ Outputting Counters: the ''counter()'' and ''counters()'' functions</h3>
 		Suggestion is to add a <css>counter-value(<<custom-ident>>)</css> function,
 		which returns the value of the named counter as an integer,
 		rather than returning a string.
+
+		See <a href="https://github.com/w3c/csswg-drafts/issues/1026">Issue 1026</a>.
 	</div>