Add note linking to codec tutorial in custom-types

Author: alcaeus

docs/tutorial/custom-types.txt

@@ -4,6 +4,12 @@ Custom Data-Types
 
 .. default-domain:: mongodb
 
+.. note::
+
+   This tutorial explains implementing custom data types using the :php:`MongoDB\\BSON\\Persistable <class.mongodb-bson-persistable>`
+   interface found in the MongoDB extension. Consider using a codec instead to decouple the MongoDB persistence logic
+   from your business logic. See the :doc:`Codec tutorial </tutorial/codecs>` for an example.
+
 The MongoDB PHP extension and library support custom classes while
 serializing and deserializing. An example of where this might be useful is
 if you want to store date/time information retaining the time zone