README.rst: Mention testing with tox

Matthias Koeppe · Zerline · commit ce06b4698e72 · 2020-05-23T09:41:41.000+02:00
diff --git a/README.rst b/README.rst
@@ -73,6 +73,10 @@ Shorthand::
 
     $ make test
 
+If you have ``tox`` installed, you can also test the package like this::
+
+    $ tox
+
 Documentation
 -------------
 
