Add explanation

sqlalchemy_utils/aggregates.py

@@ -198,6 +198,9 @@ to define lots of relationships pointing to same class, remember to define the r
 Multi-level aggregates
 ----------------------
 
+Aggregates can span accross multiple relationships. In the following example each
+Catalog has a net_worth which is the sum of all products in all categories.
+
 
 ::
 
@@ -236,6 +239,7 @@ Multi-level aggregates
         category_id = sa.Column(sa.Integer, sa.ForeignKey(Category.id))
 
 
+
 TODO
 ----