Add note about using protocol v3 with nested collections

cassandra/util.py

@@ -576,7 +576,7 @@ class OrderedMap(Mapping):
         ['value', 'value2']
 
     These constructs are needed to support nested collections in Cassandra 2.1.3+,
-    where frozen collections can be specified as parameters to others::
+    where frozen collections can be specified as parameters to others\*::
 
         CREATE TABLE example (
             ...
@@ -587,6 +587,10 @@ class OrderedMap(Mapping):
     This class dervies from the (immutable) Mapping API. Objects in these maps
     are not intended be modified.
 
+    \* Note: Because of the way Cassandra encodes nested types, when using the
+    driver with nested collections, :attr:`~.Cluster.protocol_version` must be 3
+    or higher.
+
     '''
     def __init__(self, *args, **kwargs):
         if len(args) > 1: