minor #1203 Add Blackfire as a sponsor (fabpot)

This PR was merged into the 6.2-dev branch.

Discussion
----------

Add Blackfire as a sponsor

<!-- Please fill in this template according to the PR you're about to submit. -->

| Q             | A
| ------------- | ---
| Bug fix?      | yes/no
| New feature?  | yes/no
| Doc update?   | yes/no
| BC breaks?    | yes/no
| Deprecations? | yes/no
| Fixed tickets | #... <!-- #-prefixed issue number(s), if any -->
| License       | MIT

<!-- Replace this comment by the description of your issue. -->

Commits
-------

71c7c23 added Blackfire as a sponsor

README.md

@@ -0,0 +1,19 @@
+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>