Fixes #6964.

Description

Add LICENSE requirement in specification.
https://github.com/Project-MONAI/model-zoo/blob/bdd183ebc113a70d50db4e4fb2ebd9a7c8c7532b/ci/verify_bundle.py#L28

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.