Add previously deleted circleci readme back to repo (#85598)
This readme was deleted here: https://github.com/pytorch/pytorch/pull/73224 I chatted with the author, who doesn't remember exactly why it was deleted but suspects it was due either to out of date contents or because of the upcoming migration to github actions.
With that said, we have references to this readme through our circleci directory, and since we do still have a lot of circleci workflows I feel this readme still adds a lot of value. (I recently did some CI tasks that required me to dig this readme up in order to solve a problem).
I recommend we restore this file with a warning that its contents may be out of date, until our CircleCI workflows are entirely migrated to Github Actions
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85598
Approved by: https://github.com/clee2000, https://github.com/malfet
