Add to README suggestion

[plandes]

I'd be glad to create a pull request, but thought it would be easier for everyone if I created an issue.

For those wanting to use Emacs Cider you have to start the REPL as a headless process with spark-submit using the Cider REPL handler.

You'll also need to add the dependency that matches the Cider version you have. You could add this in a lein profile like I do, which is to create an uberjar that gets deployed to an AWS EMR yarn cluster with:

(defproject ...
  :profiles {:aws-deploy {:dependencies [[cider/cider-nrepl "0.14.0"]]}})

[cgrand]

PR welcome, thanks!