Missing documentation about this repo and recent edxapp changes

[robrap]

• The README and docs directory is sorely lacking. I'm not sure where there is history for this repo.
• This repo was created for edx-exams to pass JWTs between LMS and another service to ensure the calls could be trusted.
• Originally, the LMS side of the equation was also using this repo, which made it more clear how this was all connected. However, because this was an edx org repo, this repo was dropped as a dependency, and instead, the relevant code was copied into the LMS.
  • See chore: Remove edx-token-utils dependency openedx/edx-platform#36077
• At a minimum, this issue captures some of the history.