[indexes] Add intro.

Author: tabatkins

indexes/Overview.bs

@@ -10,6 +10,30 @@ ED: https://drafts.csswg.org/indexes/
 Abstract: A listing of every term defined by CSS specs.
 </pre>
 
+Introduction {#intro}
+=====================
+
+This is a non-normative document.
+It lists every term defined in CSS,
+as long as it's marked up correctly for the spec generator.
+It is intended as a reference document for authors, implementors, and spec authors,
+to aid in finding the definition for a term
+hidden somewhere in the haystack of CSS specs.
+
+There are some things missing,
+and there are several mistakes,
+either from use of legacy markup patterns that haven't been udpated yet,
+or the dfn-type-autodetector misfiring.
+These will be cleaned up over time.
+
+Issue: A notable absence at the moment is a list of all the "dfn" type definitions--
+the ordinary words and phrases given meaning in CSS.
+This will show up shortly,
+as soon as I add a mechanism to limit the index generation
+to specs from a particular group.
+(Right now you have to either list every spec you want explicitly,
+or draw from all specs.)
+
 Properties and Descriptors {#properties}
 ========================