DOCSP-43525 - Decimal128 (#240)

Author: mongoKart

source/upgrade/v3.txt

@@ -106,4 +106,13 @@ Version 3.0 Breaking Changes
   :ref:`csharp-serialization` guide.
 
 - TLS 1.0 and 1.1 are no longer supported. You must use TLS 1.2 or higher. To learn
-  more about configuring TLS/SSL in the {+driver-short+}, see :ref:`<csharp-tls>`.
+  more about configuring TLS/SSL in the {+driver-short+}, see :ref:`<csharp-tls>`.
+
+- By default, the driver serializes ``Decimal128`` and ``decimal`` values as BSON
+  ``Decimal128`` values. In previous versions of the driver, the driver serialized these
+  values as BSON ``string`` values by default.
+  To serialize a ``decimal`` or ``Decimal128`` value as a string in v3.0, 
+  apply the ``[BsonRepresentation(BsonType.String)]`` attribute to the field.
+  
+  To learn more about specifying BSON types during serialization, see the
+  :ref:`Custom Serialization <csharp-custom-serialization>` section of the POCOs page.