Readme fix related to PSR-0 class loader

Hello,

I've added today Requests to an existing app that already uses the Symfony Class Loader.

According to the documentation I have to use:

```
$loader->registerNamespace('Requests', 'path/to/vendor/Requests/library');
```

However this doesn't work, as Requests doesn't use namespaces. I figured out that the proper way to initialize Requests with the class loader is to use the `registerPrefix` method instead:

```
$loader->registerPrefix('Requests', 'path/to/vendor/Requests/library');
```

I fixed it in the README