[css-inline-3] Add link to hanging-punctuation issue.

Author: fantasai

css-inline-3/Overview.bs

@@ -1190,7 +1190,10 @@ Edge Effects: Indentation and Hanging Punctuation</h4>
 		<img src="images/InitialLetterWithTextIndent.png" width="300"
 		     alt="initial letter with text indent">
 		<figcaption>Initial letter with text indent. </figcaption>
-	</figure>b
+	</figure>
+
+	ISSUE: The interaction of 'initial-letter' and 'hanging-punctuation'
+	is <a href="hhttps://github.com/w3c/csswg-drafts/issues/310#issuecomment-396765893">under discussion</a>.
 
 <h4 id="initial-letter-ancestors">
 Ancestor Inlines</h4>