Added Derived Class count query

Sample rules/X++/DerivedClassCount.xq

@@ -0,0 +1,16 @@
+(: Copyright (c) Microsoft Corporation.
+   Licensed under the MIT license. :)
+
+(: List classes with the number of derived classes :)
+(: @Category Informational :)
+(: @Language Xpp :)
+
+<DerivedClassCounts>
+{
+    for $c in /Class
+    let $count := count(/Class[@Extends=$c/@Name])
+    where $count != 0
+    order by $count descending
+    return <DerivedClasses Name='{$c/@Name}' Count='{$count}' />
+}
+</DerivedClassCounts>