Update usage information for anonymizer

[sanashah007]

specify what to do when converting from Dicom to Nifti

Fixes #715

Proposed Changes

• Changed Usage.md documentations to specify what to do when using gandlf_anonymizer to convert from Dicom to Nifti

Checklist

• I have read the CONTRIBUTING guide.
• My PR is based from the current GaNDLF master .
• Non-breaking change (does not break existing functionality): provide as many details as possible for any breaking change.
• Function/class source code documentation added/updated.
• Code has been blacked for style consistency.
• If applicable, version information has been updated in GANDLF/version.py.
• If adding a git submodule, add to list of exceptions for black styling in pyproject.toml file.
• Usage documentation has been updated, if appropriate.
• Tests added or modified to cover the changes; if coverage is reduced, please give explanation.
• If customized dependency installation is required (i.e., a separate pip install step is needed for PR to be functional), please ensure it is reflected in all the files that control the CI, namely: python-test.yml, and all docker files [1,2,3].

[github-actions]

MLCommons CLA bot All contributors have signed the MLCommons CLA ✍️ ✅

[sarthakpati]

LGTM

[sarthakpati]

Thanks for the PR @sanashah007! Please drop a comment when the CLA is done from your end and this can be merged).

[sanashah007]

CLA is done!