Add Blackfire as a sponsor

README.md

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+Swift Mailer
+------------
+
+Swift Mailer is a component based mailing solution for PHP 7.
+It is released under the MIT license.
+
+Swift Mailer is highly object-oriented by design and lends itself
+to use in complex web application with a great deal of flexibility.
+
+For full details on usage, read the [documentation](https://swiftmailer.symfony.com/docs/introduction.html).
+
+Sponsors
+--------
+
+<div>
+    <a href="https://blackfire.io/docs/introduction?utm_source=swiftmailer&utm_medium=github_readme&utm_campaign=logo">
+        <img src="https://static.blackfire.io/assets/intemporals/logo/png/blackfire-io_secondary_horizontal_transparent.png?1" width="255px" alt="Blackfire.io">
+    </a>
+</div>