docs: added more details to the text-processor README.md and tests

[JoshuaKGoldberg]

PR Checklist

• [x] Addresses an existing open issue: fixes #199
• [x] That issue was marked as accepting prs
• [x] Steps in CONTRIBUTING.md were taken

Overview

As I've realized re-reading #199 and the project, the README description is pretty sparse. This adds more details to it.