update read me - how to specify file name.

The `file` method is for downloading files from the disk - http://book.cakephp.org/2.0/en/controllers/request-response.html#sending-files

Whereas the `download` method is for dealing with dynamically generated content - http://book.cakephp.org/2.0/en/controllers/request-response.html#sending-a-string-as-file

So I've documented using `download` method to specify file name.

Have fun CakeFesting!

Signed-off-by: Joshua Paling <joshua.paling@gmail.com>