Added Cider's .nrepl-port file.

[Cider](https://github.com/clojure-emacs/cider) is a Clojure IDE and REPL for Emacs which uses [nrepl](https://github.com/clojure/tools.nrepl) to communicate with a remote REPL. The `.nrepl-port` file contains the port of the current nrepl connection and it will exist when Cider is running.