Update README.md

Added Mautic

README.md

 * [Stampie](https://github.com/henrikbjorn/Stampie) - A library for email services such as [SendGrid](http://sendgrid.com), [PostMark](https://postmarkapp.com), [MailGun](http://www.mailgun.com) and [Mandrill](http://www.mandrill.com).
 * [CssToInlineStyles](https://github.com/tijsverkoyen/CssToInlineStyles) - A library to inline CSS in email templates.
 * [Email Validator](https://github.com/nojacko/email-validator) - A small email address validation library.
+* [Mautic](https://github.com/mautic/mautic) - Email marketing automation
 
 ## Files
 *Libraries for file manipulation and MIME type detection.*