README.md - updating for newer version of vagrant and vagrant-libvirt plugin

Tested with these versions while doing a new configuration.  It sounded like
others may also be using these versions.
Adds the ability to define custom boxes locally.
Merge pull request #79 from ehelms/custom-boxes

Adds the ability to define custom boxes locally.
handle missing boxes.yaml
Merge pull request #83 from jlsherrill/fix_file

handle missing boxes.yaml
Merge pull request #78 from bbuckingham/update-readme

README.md - updating for newer version of vagrant and vagrant-libvirt plugin