[css-text-decor-4] Add issue.

Author: fantasai

css-text-decor-4/Overview.bs

@@ -970,6 +970,10 @@ Inset Edges: the 'text-decoration-skip-inset' property</h4>
 			</div>
 	</dl>
 
+	ISSUE: This might want to be a standalone property rather than part of the 'text-decoration-skip' set.
+	See also <a href="https://github.com/w3c/csswg-drafts/issues/4557">Issue 4557</a>,
+	about controlling the line length explicitly.
+
 <h4 id="text-decoration-skip-spaces-property">
 Skipping Spaces: the 'text-decoration-skip-spaces' property</h4>