Add the capistrano v3 way of using gateway servers

This is based on the discussion in
https://github.com/capistrano/capistrano/issues/725#issuecomment-30089068
and there is also the git config way of doing it.

I'm not sure if those were the best places to document and either if we should make a small edit in https://github.com/capistrano/documentation/blob/master/_posts/2013-06-01-release-announcement.markdown or not.