Add documentation for all box_* config options

[cornfeedhobo]

I'd like to add support for pulling boxes from an alternative location (private vagrant cloud, etc)

[cornfeedhobo]

Correction. I now see that there is already support, it's just not documented in the README. I'll submit a PR if I get a chance.

[themalkolm]

What exactly do you want to document?

[cornfeedhobo]

e.g. box_url

[themalkolm]

ok, good point :)