Added an example of how to use cond

source/reference/aggregation/cond.txt

@@ -11,9 +11,26 @@ $cond (aggregation)
    .. code-block:: javascript
 
       { $cond: [ <boolean-expression>, <true-case>, <false-case> ] }
-
+      
    Takes an array with three expressions, where the first expression
    evaluates to a Boolean value. If the first expression evaluates to true,
    :expression:`$cond` returns the value of the second expression. If the
    first expression evaluates to false, :expression:`$cond` evaluates and
    returns the third expression.
+
+
+   .. example::
+
+      The following aggregation returns the number of documents where the
+      amount field is greater than 100.
+
+      .. code-block:: javascript
+
+         db.accounts.aggregate([
+         { 
+            $group: { 
+               _id: "totals",
+               totalAmount:   { $sum: "$amount" },
+               maxAmount:     { $max: "$amount" },
+               overMaxCount:  { $sum: { $cond: [ { $gt: [ "$amount", 100 ] }, 1, 0 ] } },
+         }} ]);