Add code example for default and format=None

docs/api-guide/fields.md

@@ -297,6 +297,46 @@ Corresponds to `django.db.models.fields.DateTimeField`.
   * Setting this value to `None` indicates that Python `datetime` objects should be returned by `to_representation`. In this case the datetime encoding will be determined by the renderer.
 * `input_formats` - A list of strings representing the input formats which may be used to parse the date.  If not specified, the `DATETIME_INPUT_FORMATS` setting will be used, which defaults to `['iso-8601']`.
 
+For example, lets have the following model:
+
+```python
+from django.db import models
+
+
+class Comment(models.Model):
+    created_at = models.DateTimeField(auto_now_add=True)
+```
+
+And the following serializers:
+
+```python
+from rest_framework import serializers
+
+
+class CommentSerializerWithDefaultDateTimeField(serializers.Serializer):
+    created_at = serializers.DateTimeField()
+
+
+class CommentSerializerWithFormatNone(serializers.Serializer):
+    created_at = serializers.DateTimeField(format=None)
+```
+
+Now we have the following types:
+
+```python
+import datetime
+
+c = Comment.objects.create()
+
+s1 = CommentSerializerWithDefaultDateTimeField(c)
+s2 = CommentSerializerWithFormatNone(c)
+
+
+assert type(s1.data['created_at']) is str
+assert type(s2.data['created_at']) is datetime.datetime
+```
+
+
 #### `DateTimeField` format strings.
 
 Format strings may either be [Python strftime formats][strftime] which explicitly specify the format, or the special string `'iso-8601'`, which indicates that [ISO 8601][iso8601] style datetimes should be used. (eg `'2013-01-29T12:34:56.000000Z'`)