Release v6.2.1 - Update RTC readme

[hjkristenson]

This PR updates the RTC readme file to remove references to standard products and more clearly describe which of the files are optional for inclusion in the product package.

[github-actions[bot]]

Developer checklist

• [x] Indicated the level of changes to this package by affixing one of these labels:
  • major -- Major changes to the API that may break current workflows
  • minor -- Minor changes to the API that do not break current workflows
  • patch -- Patches and bugfixes for the current version that do not break current workflows
  • bumpless -- Changes to documentation, CI/CD pipelines, etc. that don't affect the software's version
• ~[ ] (If applicable) Updated the dependencies and indicated any downstream changes that are required~
• [x] Added/updated documentation for these changes
• ~[ ] Added/updated tests for these changes~

Reviewer checklist

• ~[ ] Have all dependencies been updated?~
• [x] Is the level of changes labeled appropriately?
• [x] Are all the changes described appropriately in CHANGELOG.md?
• [x] Has the documentation been adequately updated?
• ~[ ] Are the tests adequate?~