README: Clarify how to choose an API version

There are many version numbers in play here. Help users choose the
correct one.

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

README.rst

@@ -79,7 +79,8 @@ settings are **required**:
 
   You can discover the API version supported by your instance by comparing the
   server version, found at ``/about``, with the API versions provided in the
-  `documentation`__.
+  `documentation`__. For example, if your server is running Patchwork version
+  2.1.x, you should use API version 1.1.
 
   __ https://patchwork.readthedocs.io/en/stable-2.1/api/rest/#rest-api-versions