Showing an explicit version of using `multifile`.

Showing how to use `src` and `dest` explicitly, to avoid having the source overridden.

I've found that using

```js
multifile : {
  files : {
    'test/test.processed.html' : 'test/test.html',
    'test/test.processed.js'   : 'test/test.js'
}
```
was overriding my src files, so I've changed the README to, imho, a safer version. It's worth pointing out that I did not have the `inline` flag set to `true`. 
Did I understand the behaviour correctly? If not I'm sorry and please close this.

Thanks for the work you did on this project.