Clarify README.md on Auto-reloading

It wasn't clear that `grunt debug` gives you auto-reloading. This should be more clear. See: #31 

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/yeoman/generator-chrome-extension/85)
<!-- Reviewable:end -->