Update the readme

The readme had me confused for a little bit. I couldn't figure out why the site wasn't being served.

I finally realized it was because I had dev-site.local in /etc/hosts, but Vagrant was really wanting to serve stuff on dev-site.vm since this is what's in the Vagrantfile.

The readme now says to add dev-site.vm to your /etc/hosts. I also included making the "public" directory as part of step 3.