Rename README.txt to README.md

[jackhenryellis]

Committer Notes

{Please provide a brief description of what this PR accomplishes. Be sure to reference any issues addressed. If the PR is a work-in-progress submitted for early review, please include [WIP] at the beginning of the title or mark the PR as DRAFT.}

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Changes to Core Features:

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[aj-stein-nist]

First of @jackhenryellis, I want to thank you for your first PR to this project. Every one counts. Unfortunately, I do not think we can accept this one. This text file has this extension to signify to the project maintainers that the text file is ingested as-is as a preface to the auto-generated release notes generated by GitHub Actions when it processes a tagged released. It is not labeled as a Markdown file with that .md extension and accidentally signify it is developer-facing, public documentation for staff and those like yourself. So we will keep it as-is, but I do appreciate the recommendation. Do not feel discouraged from future contributions, big or small, but this file has the .txt extension as intended.