Adding FAQ for automatically setting environment variables

I really appreciate how easy Boot2Docker is to use, and how you all stay on top of the latest Docker releases.

Boot2Docker is correct to not force users to automatically set the environment variables for Docker clients. I work with Docker and boot2docker often, though, so I wrote a small script this afternoon that does automatically set the Boot2Docker environment variables: https://github.com/atbaker/boot2docker-autoset

This PR adds an FAQ about automatically setting the environment variables and points users to the boot2docker-autoset repo.

boot2docker-autoset is a *very* small script, but I figured including it in the FAQs might help other devs who want to automatically set the Boot2Docker environment variables in each session.

Thanks!