update posframe panel usage comment

;; To use this panel, install posframe package, then add in your

;; ~/.emacs.d/init.el file,

;;

;;   (require 'zero-input)

;;   ;; other user configurations

;;   (when (locate-library "posframe")

;;     (require 'zero-input-panel-posframe)

;;     (zero-input-panel-posframe-init))