Add new section to README to credit supporters (#3394)

Credits only BrowserStack for now.

README.rst

@@ -70,6 +70,14 @@ Commercial Support
 ~~~~~~~~~~~~~~~~~~
 Wagtail is sponsored by `Torchbox <https://torchbox.com/>`_. If you need help implementing or hosting Wagtail, please contact us: hello@torchbox.com.
 
+Thanks
+~~~~~~
+We thank `BrowserStack <https://www.browserstack.com/>`_, who provide the project with free access to their live web-based browser testing tool, and automated Selenium cloud testing.
+
+.. image:: https://cdn.rawgit.com/springload/wagtail/docs/project-sponsors/.github/browserstack-logo.svg
+    :target: https://www.browserstack.com/
+    :width: 219 px
+
 Compatibility
 ~~~~~~~~~~~~~
 Wagtail supports Django 1.8.1+ on Python 2.7, 3.3, 3.4 and 3.5. Supported database backends are PostgreSQL, MySQL and SQLite.