[css-inline-3] Add issue referencing open discussion about initial letter vs. inline-end floats.

Author: fantasai

css-inline-3/Overview.bs

@@ -1287,6 +1287,13 @@ Interaction with floats</h4>
 	and adjacent content.
 	[[!CSS2]]
 
+	ISSUE: Whether an inline-end float originating in subsequent lines
+	must clear the initial letter (as inline-start floats do)
+	is <a href="https://lists.w3.org/Archives/Public/www-style/2018Jul/0019.html">still under discussion</a>.
+	There is no aesthetic reason to require it;
+	however it’s yet unclear how the underlying layout model
+	would distinguish between the two cases.
+
 <h4 id="intial-letter-breaking">
 Interaction with Fragmentation (Pagination)</h4>