fix(docs): update testing with ddtest and riot (DataDog#2575)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
Co-authored-by: Julien Danjou <julien.danjou@datadoghq.com>

README.md

@@ -62,24 +62,26 @@ launch them through:
 
 #### Running Tests in docker
 
-Once your docker-compose environment is running, you can run the test runner image:
-
-    $ docker-compose run --rm testrunner
-
-Now you are in a bash shell. You can now run tests as you would do in your local environment:
-
-    $ tox -e '{py35,py36}-redis{210}'
-
-We also provide a shell script to execute commands in the provided container.
-
-For example to run the tests for `redis-py` 2.10 on Python 3.5 and 3.6:
-
-    $ ./scripts/ddtest tox -e '{py35,py36}-redis{210}'
-
-If you want to run a list of tox environment (as CircleCI does) based on a
-pattern, you can use the following command:
-
-    $ scripts/ddtest scripts/run-tox-scenario '^futures_contrib-'
+Once your docker-compose environment is running, you can use the shell script to
+execute tests within a Docker image. You can start the container with a bash shell: 
+
+    $ scripts/ddtest
+
+You can now run tests as you would do in your local environment. We use
+[tox][tox] as well as [riot][riot], a new tool that we developed for addressing
+our specific needs with an ever growing matrix of tests. You can list the tests
+managed by each:
+
+    $ tox -l
+    $ riot list
+
+You can run multiple tests by using regular expressions:
+
+    $ scripts/run-tox-scenario '^futures_contrib-'
+    $ riot run psycopg
+
+[tox]: https://github.com/tox-dev/tox/
+[riot]: https://github.com/DataDog/riot/
 
 ### Continuous Integration