[css-ui-4] Add an issue about appearance: button being controversial

Author: frivoal

css-ui-4/Overview.bs

@@ -328,6 +328,11 @@ so we may want to include it as well.
 	<dt><dfn>button</dfn>
 	<dd>The element is rendered with the look and feel of a push button,
 	similar to the rendering of the HTML <{button}> element.
+
+	Issue: There are strong concerns as to whether this property should have any value
+	other than ''appearance/auto'' and  ''appearance/none''.
+	The ''appearance/button'' value may be necessary due to backward compatibility concerns,
+	but if it is not, there is a strong possibility that it will be dropped.
 </dl>
 
 On form control elements where the computed value is ''appearance/auto''