README: $terminfo does not work under Ubuntu 12.04

This gives further advice for keybindings: for some reason, on my Ubuntu
12.04 system, I need to enter `^[[A` but `$terminfo[kcuu1]` gives `^[OA`
README: document that ZSH 4.3 or newer is required

closes #24
Merge branch 'master' of https://github.com/zsh-users/zsh-history-substring-search

* 'master' of https://github.com/zsh-users/zsh-history-substring-search:
  README: document that ZSH 4.3 or newer is required
  README: $terminfo does not work under Ubuntu 12.04