Point out it's a zero-dependency package in README

More and more of the community seems to be encouraging use of
zero-dependency packages. We might as well point that out early
on in our README since that's how mustache.js always has been
and planned to be going forward.

README.md

@@ -4,7 +4,7 @@
 
 [![Build Status](https://travis-ci.org/janl/mustache.js.svg?branch=master)](https://travis-ci.org/janl/mustache.js) [![Gitter chat](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/janl/mustache.js)
 
-[mustache.js](http://github.com/janl/mustache.js) is an implementation of the [mustache](http://mustache.github.com/) template system in JavaScript.
+[mustache.js](http://github.com/janl/mustache.js) is a zero-dependency implementation of the [mustache](http://mustache.github.com/) template system in JavaScript.
 
 [Mustache](http://mustache.github.com/) is a logic-less template syntax. It can be used for HTML, config files, source code - anything. It works by expanding tags in a template using values provided in a hash or object.