Update README.md

Looks like you updated the grunt task name from `cache-bust` to `cacheBust` based on the help output:


    $ grunt --help | grep bust
    cacheBust  Add a hash as a query string parameter on static assets *
ignore js/css/img hosted on a remote server

- cacheBust should ignore files hosted on a CDN as these files typically
  include version strings to identify code changes

- including a hash value in the querystring at best causes a cache miss
  in the browser, causing the browser to redownload already cached
  version of CDN hosted libraries like jQuery, Bootstrap, or AngularJS;
  at worst, the remote server might alter the response based on a the
  querystring parameter

- the current cacheBust implementation does not accurately hash remote
  URLs, as it would need to download the URL to a local file first
Merge pull request #3 from steve-jansen/master

ignore js/css/img hosted on a remote server