Add Mandrill to list of libraries that Stampie can interface with.

README.md

 * [PHPMailer](https://github.com/PHPMailer/PHPMailer) - Another mailer solution.
 * [Fetch](https://github.com/tedivm/Fetch) - An IMAP library.
 * [Email Reply Parser](https://github.com/willdurand/EmailReplyParser) - An email reply parser library.
-* [Stampie](https://github.com/henrikbjorn/Stampie) - A library for email services such as [SendGrid](http://sendgrid.com), [PostMark](http://postmarkapp.com) and [MailGun](http://www.mailgun.com).
+* [Stampie](https://github.com/henrikbjorn/Stampie) - A library for email services such as [SendGrid](http://sendgrid.com), [PostMark](http://postmarkapp.com), [MailGun](http://www.mailgun.com) and [Mandrill](http://www.mandrill.com).
 
 ## Files
 *Libraries for file manipulation and MIME type detection.*