Pundit: Add namespaces support to documentation

[guzart]

Namespaces support was added on #331, however the documentation for that features is missing in the README.

[jnicklas]

Yes, we definitely should document this. Do you want to send a PR?

[JenHomann]

Is this issue just waiting for a PR with passing tests, as it doesn't look like #410 is currently passing? If so, happy to do my part and get the documentation to reflect this (very helpful) behavior.

[JenHomann]

On closer inspection, it seems like the failures here were caused a random infrastructure blip. Can we trigger them to run again, @guzart? If more work is needed at that point, I'd be glad to work on it if need be.

[guzart]

Thanks @JenHomann, I've triggered a new build. Test are passing now.
It's worth mentioning that besides #410 there's also #392.

Note: I won't feel offended if #392 is merged 😉