Added example of using default/object_hook into README

README.rst

@@ -61,6 +61,36 @@ stream.
        for unpacked in unpacker:
            print unpacked
 
+packing/unpacking of custom data type
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Also possible to pack/unpack user's data types. Here is an example for
+``datetime.datetime``.
+
+::
+    import datetime
+
+    import msgpack
+
+    useful_dict = {
+        "id": 1,
+        "created": datetime.datetime.now(),
+    }
+
+    def decode_datetime(obj):
+        if b'__datetime__' in obj:
+            obj = datetime.datetime.strptime(obj["as_str"], "%Y%m%dT%H:%M:%S.%f")
+        return obj
+
+    def encode_datetime(obj):
+        if isinstance(obj, datetime.datetime):
+            return {'__datetime__': True, 'as_str': obj.strftime("%Y%m%dT%H:%M:%S.%f")}
+        return obj
+
+
+    packed_dict = msgpack.packb(useful_dict, default=encode_datetime)
+    this_dict_again = msgpack.unpackb(packed_dict, object_hook=decode_datetime)
+
 
 INSTALL
 ---------