Added missing instructions to caveats

In the openssh.rb recipe, to properly use --with-keychain-support, it is necessary to run and quit ssh-agent per each login shell, and I updated the caveat with a few lines to add to one's ~/.bash_profile to handle this gracefully and invisibly.  Without those lines, openssh will store private key passwords in your keychain, but fail to recall them for use (which kind of defeats the purpose). There are other ways that don't require adding anything to your dotfiles, but they require additional alteration of System configuration files and doing it on a per-user basis via ones own .bash_profile is the least invasive method, in my opinion.