* Allow specifying the desired IO selector backend
nio4r will soon have experimental support for io_uring via libev. The brave
might want to try it out :) regardless of that, allowing users to explicitly
set the IO selector backend seems like a good idea to me.
* Change History.md
* Add PR number
* Fix History.md
Oops!
* Update docs based on PR feedback
* Add test serving requests using a custom IO selector
* Document the NIO4R_PURE environment variable
* Raise clearer error when the requested backend is unsupported
* Simply boot the server and check the backend
