https://github.com/mustache/emacs
In your shell:
cd ~/.emacs.d/vendor
curl -O https://github.com/mustache/emacs/raw/master/mustache-mode.el
In your Emacs config:
(add-to-list 'load-path "~/.emacs.d/vendor/mustache-mode.el")
(require 'mustache-mode)
This package is now available on MELPA, and I'd strongly recommend installing it there. It's easy, it works, it's integrated nicely. http://melpa.milkbox.net/#/mustache-mode