Replace sponsor HTML with markdown

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

Author: Ryan P Kilby

.travis.yml

@@ -22,7 +22,10 @@ matrix:
       - { python: "3.6", env: DJANGO=2.0 }
       - { python: "2.7", env: TOXENV=lint }
       - { python: "2.7", env: TOXENV=docs }
-      - { python: "2.7", env: TOXENV=readme }
+      - python: "2.7"
+        env: TOXENV=readme
+        addons:
+          apt_packages: pandoc
     exclude:
       - { python: "2.7", env: DJANGO=master }
       - { python: "2.7", env: DJANGO=2.0 }

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>
+[![][rover-img]][rover-url]
+[![][sentry-img]][sentry-url]
+[![][stream-img]][stream-url]
+[![][machinalis-img]][machinalis-url]
+[![][rollbar-img]][rollbar-url]
 
 *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/).*
 
@@ -188,6 +186,18 @@ Send a description of the issue via email to [rest-framework-security@googlegrou
 [funding]: https://fund.django-rest-framework.org/topics/funding/
 [sponsors]: https://fund.django-rest-framework.org/topics/funding/#our-sponsors
 
+[rover-img]: https://raw.githubusercontent.com/encode/django-rest-framework/master/docs/img/premium/rover-readme.png
+[sentry-img]: https://raw.githubusercontent.com/encode/django-rest-framework/master/docs/img/premium/sentry-readme.png
+[stream-img]: https://raw.githubusercontent.com/encode/django-rest-framework/master/docs/img/premium/stream-readme.png
+[machinalis-img]: https://raw.githubusercontent.com/encode/django-rest-framework/master/docs/img/premium/machinalis-readme.png
+[rollbar-img]: https://raw.githubusercontent.com/encode/django-rest-framework/master/docs/img/premium/rollbar-readme.png
+
+[rover-url]: http://jobs.rover.com/
+[sentry-url]: https://getsentry.com/welcome/
+[stream-url]: https://getstream.io/try-the-api/?utm_source=drf&utm_medium=banner&utm_campaign=drf
+[machinalis-url]: https://hello.machinalis.co.uk/
+[rollbar-url]: https://rollbar.com/
+
 [oauth1-section]: http://www.django-rest-framework.org/api-guide/authentication/#django-rest-framework-oauth
 [oauth2-section]: http://www.django-rest-framework.org/api-guide/authentication/#django-oauth-toolkit
 [serializer-section]: http://www.django-rest-framework.org/api-guide/serializers/#serializers