Add a description of how to download model in model zoo

[yiheng-wang-nv]

This feature request comes from:
Project-MONAI/model-zoo#321

[wyli]

I think we need more documentation about how to start using the model zoo, maybe with descriptions as well as the CLI commands, and prerequisites in general such as monai/pytorch installation, have the tools to unzip the files... after clicking the download button, a zip file is downloaded, it's unclear what's next.