Added child_enpoints_model example.

As per https://github.com/GoogleCloudPlatform/endpoints-proto-datastore/issues/110#issuecomment-60832527 I have added the example for simple parent child relationship.
What is NDS?

Also, you should point out that `id` is not in the default schema for `MyModel`. The phrase "we do not include it in request body" is a bit unclear.