Adding cert and ssl_key to documentation

The cert option (https://github.com/guzzle/guzzle/blob/master/src/Guzzle/Http/Message/RequestFactory.php#L341) and ssl_key option (https://github.com/guzzle/guzzle/blob/master/src/Guzzle/Http/Message/RequestFactory.php#L351) exist but aren't documented.

Simply adding to the docs with code examples.