Added boxen upgrade instructions
Added more detailed update process
Moved Upgrading instructions to FAQ
fixed spelling error
Update README.md

Other boxem fixed.
Merge pull request #587 from nbmorgan/master

Added boxen upgrade instructions
Upgrade puppet-git to 2.3.1

Closes #598
Merge branch 'master' of https://github.com/boxen/our-boxen
Updated both gems and puppet modules.