Add an example of "how to make a file public" in the docs

I've heard from a lot of people that this is the most common request, so it'd be really cool if we had an example in our docs.

ie:

---

To make an object publicly readable, you might do:

``` nodejs
var storage = gcloud.storage();
var albums = storage.bucket('albums');

albums.acl.add({
  scope: 'allUsers',
  permission: Storage.acl.READER_ROLE
}, function(err, aclObject) {});
```
