More explicit setup instructions needed

[kirkmadera]

To make this more accessible, we should add more detailed instructions on how to set up the VM using Vagrant along with common pitfalls. I ran into an issue that Vagrant could not find a provider and it ended up being that Virtual Box was not installed. I had to install VirtualBox and then restart my computer before it worked.

I think a list of prerequisites would be sufficient to solve this - Vagrant, Git Bash (for Windows users), and VirtualBox.

The instructions should also include this link for setting up the repo.magento.com http auth creds: http://devdocs.magento.com/guides/v2.0/install-gde/prereq/connect-auth.html