Add tip for remapping C-s

You mentioned your C-s mapping on the Ruby Rogues podcast and it sounded useful. I checked your `.vimrc` and added the mappings, but it didn't work, and I couldn't figure out why. I thought maybe iTerm was using that mapping for something else.

I later realised that it's because C-s is traditionally used for flow control, and your `.zshrc` has entries to disable that behaviour.

Since there's a lot of interest in your dotfiles, I thought it was worth adding a tip to avoid others running into the same problem.