Provide some high level text about the catalog changes

doc/WHATSNEW.rst

@@ -98,6 +98,31 @@ refactored to match this new reality. The goal is to finally remove the Five
 integration layer and make the Zope Toolkit a normal integral part of Zope 2.
 
 
+ZCatalog
+--------
+
+The ZCatalog and the default set of indexes as found in the PluginIndexes
+package have seen a large number of changes. Most of these have been pioneered
+in add-on packages in the Zope community over the last years and now have found
+their way back into the core. The largest change is added query plan support for
+the catalog. A standard feature in all relation databases, the job of a query
+plan is to monitor queries in a live system and based on execution metrics
+devise optimized plans for executing the low level instructions which lead to a
+query result. In sites with large number of indexed objects this can make a
+tremendous difference and significantly speed up all queries.
+
+The query plan support is completely transparent to all users, though ways exist
+for developers to predefine it and store it across server restarts. The plan
+itself can be introspected in a tab in the ZMI. There's also a new ZMI tab to
+report slow catalog queries which can help developers to tune the remaining slow
+queries in their applications.
+
+In addition to these larger changes there's been a high number of smaller
+changes to the search logic and the catalog implementations. All of these
+should result in better query execution and reduced number of conflict error
+potential.
+
+
 Refactoring
 -----------