adapt image to sponsor format to .rst style (#4965)

plain html broke the README

README.rst

@@ -12,9 +12,11 @@
 Sponsors
 ========
 
-<a href="https://getstream.io/try-the-api/?utm_source=celery&utm_medium=banner&utm_campaign=github" >
-   <img src="https://i.imgur.com/ULmQEib.png" alt="Stream" width="200" style="padding: 10px"/>
-</a>
+|ImageLink|
+
+.. |ImageLink| image:: https://i.imgur.com/ULmQEib.png
+.. _ImageLink: https://getstream.io/try-the-api/?utm_source=celery&utm_medium=banner&utm_campaign=github
+
 
 What's a Task Queue?
 ====================