Update README.md

install >= 2.0.1 version of vagrant-berkshelf, otherwise, it will report error as following:

cannot load such file -- hashie/hash_extensions
renamed public
Merge pull request #26 from ilyar/patch-1

renamed public
Merge pull request #23 from caoer/patch-1

Update README.md
site replaced to source
added environment variables
Merge branch 'replaced_site_location'