Adds toString explanation

I noticed that there is a toString() function at https://github.com/dart-lang/one-hour-codelab/blob/master/web/4-classbadge/piratebadge.dart#L57, but isn't mentioned in the tutorial, so I added a brief explanation.
Thanks for catching this! I've confirmed that your text displays properly, but I'd like to tweak it.

First, we should probably emphasize that you're overriding the method, not simply adding a function. Second, we could explain more about why toString() is important. Maybe something like this:

```
Override the toString() method.
...
* Because the Object implementation of `toString()` doesn't give much information,
  many classes override `toString()`.
* When you call `print(anObject)` for any non-String, it prints the value returned
  by `anObject.toString()`.
* Overriding `toString()` can be especially helpful for debugging or logging.
```

What do you think? If you like the direction of these changes but have improvements, go ahead and make them. Or I can merge your pull request and then make these changes before publishing the site.

Again, thanks!