Add auto-mode-alist with autoload cookie

Hi, I plan to add this to [MELPA](http://melpa.milkbox.net/). This commit ensures that users who have installed `union-mode` from [MELPA](http://melpa.milkbox.net/)  will be able to automatically turn on `union-mode` when visiting a `prf` file without explicitly requiring the library first. For information about this fix [Packaging-Basics](http://www.gnu.org/software/emacs/manual/html_node/elisp/Packaging-Basics.html).