Please update Readme regarding higher-order effects and NonDet/Coroutine

[ymdryo]

The README.md currently contains the following statement:

It needs to be noted however that such NonDet effect handler in existing libraries is broken and none of the ones with support for higher order effects provide the Coroutine effect, so arguably it's not a big loss.

It seems that my library is currently the only one that supports both without broken semantics. I would be grateful if you could consider updating this information.

[arybczak]

Thanks for the suggestion. I plan to reorganize the README at some point in the near future, I'll do it then.