Replace sponsor HTML with markdown

This loses the image centering, but can be converted to RST.

Author: Ryan P Kilby

README.md

@@ -20,13 +20,11 @@ continued development by **[signing up for a paid plan][funding]**.
 The initial aim is to provide a single full-time position on REST framework.
 *Every single sign-up makes a significant impact towards making that possible.*
 
-<p align="center">
-  <a href="http://jobs.rover.com/"><img src="https://raw.githubusercontent.com/encode/django-rest-framework/master/docs/img/premium/rover-readme.png"/></a>
-  <a href="https://getsentry.com/welcome/"><img src="https://raw.githubusercontent.com/encode/django-rest-framework/master/docs/img/premium/sentry-readme.png"/></a>
-  <a href="https://getstream.io/try-the-api/?utm_source=drf&utm_medium=banner&utm_campaign=drf"><img src="https://raw.githubusercontent.com/encode/django-rest-framework/master/docs/img/premium/stream-readme.png"/></a>
-  <a href="https://hello.machinalis.co.uk/"><img src="https://raw.githubusercontent.com/encode/django-rest-framework/master/docs/img/premium/machinalis-readme.png"/></a>
-  <a href="https://rollbar.com/"><img src="https://raw.githubusercontent.com/encode/django-rest-framework/master/docs/img/premium/rollbar-readme.png"/></a>
-</p>
+[![](https://raw.githubusercontent.com/encode/django-rest-framework/master/docs/img/premium/rover-readme.png)](http://jobs.rover.com/)
+[![](https://raw.githubusercontent.com/encode/django-rest-framework/master/docs/img/premium/sentry-readme.png)](https://getsentry.com/welcome/)
+[![](https://raw.githubusercontent.com/encode/django-rest-framework/master/docs/img/premium/stream-readme.png)](https://getstream.io/try-the-api/?utm_source=drf&utm_medium=banner&utm_campaign=drf)
+[![](https://raw.githubusercontent.com/encode/django-rest-framework/master/docs/img/premium/machinalis-readme.png)](https://hello.machinalis.co.uk/)
+[![](https://raw.githubusercontent.com/encode/django-rest-framework/master/docs/img/premium/rollbar-readme.png)](https://rollbar.com/)
 
 *Many thanks to all our [wonderful sponsors][sponsors], and in particular to our premium backers, [Rover](http://jobs.rover.com/), [Sentry](https://getsentry.com/welcome/), [Stream](https://getstream.io/?utm_source=drf&utm_medium=banner&utm_campaign=drf), [Machinalis](https://hello.machinalis.co.uk/), and [Rollbar](https://rollbar.com/).*