Add description metadata to model info

[RomanFenioux]

Description

This PR adds the description metadata to the model info file, as described in the issue #770

The rationale is to have access to more information about the available model when using https://api.github.com/repos/Project-MONAI/model-zoo/releases/tags/hosting_storage_v1

Please let me know your thought, and if this is the right approach to solve this issue.

Status

Ready