Add information on the maintainer of the external files linked from the model zoo

[tvercaut]

It would be good to add some information about who is maintaining (e.g. by giving an email, GitHub handle, etc.) the external files linked from a model in the model zoo, e.g.:
https://github.com/NifTK/NiftyNetModelZoo/blob/master/anisotropic_nets_brats_challenge_model_zoo.ini

Especially since the typical use case for getting models is not to clone
https://github.com/NifTK/NiftyNetModelZoo, I would suggest adding an external_files_maintainer key or similar in the main [config] section of the model file.

If we think that the use case of a different maintainer for each file is of sufficient interest (e.g. someone sharing new weights for an existing network architecture contributed by someone else), such a key could be moved/replicated to each of the [data] / [weights] / [network_inference_config] sections