doc for connecting remote repl from heroku through ssh tunnel

https://devcenter.heroku.com/articles/exec#environment-variables
as you can see here, heroku doesn't apply environment variable when using ssh tunnel.
so if you set `NREPL_PORT` through `heroku config` it will be disabled and remote repl doesn't work.

i think it would be nice to have a guide about this. such as adding `:nrepl-port` config in `prod.edn`