[css-inline-3] Add issue about hanging punctuation

Author: fantasai

css-inline-3/Overview.bs

@@ -608,6 +608,13 @@ Layout Model</h4>
 			(see 'hanging-punctuation')--
 			as well as the margin boxes of any atomic inlines it contains.
 
+			ISSUE: Should the hanging punctuation be included in the box instead
+			(so that the box is drawn around the punctuation when it is made visible through borders/background),
+			but rather only excluded when positioning the box
+			(so that the initial letter remains flush,
+			with the hanging punctuation properly hanging)?
+			See <a href="https://github.com/w3c/csswg-drafts/issues/310">discussion</a>.
+
 		<li>
 			The initial letter's margin box is placed at the edge of the line.
 			If it has no padding or borders, it is negatively offset