Document direnv

We currently use either [figaro](https://github.com/laserlemon/figaro) or [dotenv](https://github.com/bkeepers/dotenv) to manage our environment variables. These are good gems, but one could argue that adding them to your Gemfile is not ideal.

I've been using [direnv](https://github.com/zimbatm/direnv/) for various purposes (like adding `./bin` to `$PATH`) and it occurred to me that this is probably a less obtrusive way of configuring my application, too.

What do you guys think?