[css-inline] Add issue wrt root inline, 'coz we really need to define it somewheres.

Author: fantasai

css-inline/Overview.bs

@@ -25,7 +25,12 @@ Link Defaults: css-fonts-3 (property) font-family
 At Risk: the 'initial-letter-wrap' property
 </pre>
 
+<h2 id="intro">Introduction</h2>
 
+This module defines the CSS Inline Layout model,
+replacing and extending the model as defined in CSS2.1.
+
+Issue: Define root inline box.
 
 <h2 id="line-height">Line Heights and Baseline Alignment</h2>