Update README.md

[cassandrabailey293]

Updated README to mention the TDF XML format, and to provide a contact for those who are seeking to learn more about implementations around it.

Proposed Changes

Checklist

• A clear description of the change has been included in this PR.
• A clear description of whether this change is a Major, Minor, Patch or cosmetic change as per the Versioning Guidelines has been included in this PR.
• All schema validation tests have been updated appropriately and are passing.
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• DRAFT MERGE ONLY: Version numbers have been updated as per the Versioning Guidelines.
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