Add example use of SerializerMethodField to docs

docs/api-guide/fields.md

@@ -328,7 +328,21 @@ This field is always read-only.
 
 ## SerializerMethodField
 
-This is a read-only field. It gets its value by calling a method on the serializer class it is attached to. It can be used to add any sort of data to the serialized representation of your object. The field's constructor accepts a single argument, which is the name of the method on the serializer to be called. The method should accept a single argument (in addition to `self`), which is the object being serialized. It should return whatever you want to be included in the serialized representation of the object.
+This is a read-only field. It gets its value by calling a method on the serializer class it is attached to. It can be used to add any sort of data to the serialized representation of your object. The field's constructor accepts a single argument, which is the name of the method on the serializer to be called. The method should accept a single argument (in addition to `self`), which is the object being serialized. It should return whatever you want to be included in the serialized representation of the object. For example:
+
+    from rest_framework import serializers
+    from django.contrib.auth.models import User
+    from django.utils.timezone import now
+
+    class UserSerializer(serializers.ModelSerializer):
+
+        days_since_joined = serializers.SerializerMethodField('get_days_since_joined')
+
+        class Meta:
+            model = User
+
+        def get_days_since_joined(self, obj):
+            return (now() - obj.date_joined).days
 
 [cite]: http://www.python.org/dev/peps/pep-0020/
 [FILE_UPLOAD_HANDLERS]: https://docs.djangoproject.com/en/dev/ref/settings/#std:setting-FILE_UPLOAD_HANDLERS