Update README.MD

[PhBastiani]

Additionnal notes : Atomic & ConcurrenVar

[nomisRev]

Thanks @PhBastiani!

This is great 👏 👏 No work on this has been done yet, but I proposed something in #248 which is preparing the docs for release. I'd love your feedback there as well!

This could become a page that in more detail explains the integrations and cancellation in the standard library, etc.
A fourth page could be added to the quickstart which covers the IO vs suspend section currently in the README.MD of Arrow Fx Coroutines. It could also include a small migration guide from IO to suspend as today was asked on Slack

[nomisRev]

Should we mention that the other concurrency data types are still available as well, but with the same names? We also added a new data type which is CircuitBreaker.

All of these will also be promoted and covered in the Arrow Fx websites sidebar. Link to Arrow Site PR

[rachelcarmena]

Global checks were cancelled because they aren't necessary. Thanks @PhBastiani !!