Improve usage example in README.md (#778)

By showing explicitly how/where `Mustache` comes from in that
specific scenario, where CommonJS is assume to be available,
either provided by Node.js or a build tool like webpack etc.

For the record, it could also be `import` if the using project
uses ES Modules, as we've recently exposed native ES Modules
support as well.

README.md

@@ -37,6 +37,8 @@ $ npm install mustache --save
 Below is a quick example how to use mustache.js:
 
 ```js
+var Mustache = require('mustache');
+
 var view = {
   title: "Joe",
   calc: function () {