Providing a multi-put example, illustrating the use of Proto generation/conversion API.

Provide an example on how to use:
EndpointsModel.ProtoModel()
EndpointsModel.ProtoCollection()
EndpointsModel.ToMessage()
EndpointsModel.FromMessage()
to generate ProtoRPC classes to use with Endpoints API.
The example is constructed as a "batch-insert" API that takes in a "proto collection" to do batch insert into NDB and return the entities inserted as another "proto collection".
I see.

This is really my fault for not showing the true value and flexibility of this library, but the library can do it as well.

You can do it with `MyModel.method()` by [specifying][1] a ["request message"][2].

```python
  @MyModel.method(request_message=MyModel.ProtoCollection(),
                  response_message=MyModel.ProtoCollection(),
                  user_required=True,
                  path='mymodel_multi',
                  name='mymodel.insert_multi')
```

[1]: https://github.com/GoogleCloudPlatform/endpoints-proto-datastore/blob/8dd27db7f76d74fd1d71fec657b2f19b81223d76/endpoints_proto_datastore/ndb/model.py#L1345
[2]: https://github.com/GoogleCloudPlatform/endpoints-proto-datastore/blob/8dd27db7f76d74fd1d71fec657b2f19b81223d76/endpoints_proto_datastore/ndb/model.py#L48