Readme fix related to PSR-0 class loader

[fulopattila122]

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

[coveralls]

Coverage decreased (-1.14%) when pulling bbc1ab6 on fulopattila122:master into dec780a on rmccue:master.

[rmccue]

Thanks! 🍰