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.
(Moving conversation here to latest diff)

The statement "we do not include it in request body" doesn't clearly indicate why this happens. Instead you could say 

```python
  # Unlike in keys_with_ancestors/main.py, we want the datastore to generate
  # child ID, therefore we do not include it in the default message fields schema
  # which determines the request body.
```