Update README.txt regarding curl prerequisite

Hi, I've updated the CloudFront installation steps at http://drupal.org/node/552934 with a note about the modules requirement of curl support in PHP, which is something I discovered while working on my own reported issue at http://drupal.org/node/595142.

I've updated the CloudFront module's README.txt with the same note about curl support in PHP, and am attaching a patch against HEAD containing this update to this ticket.

Thanks!