Add documentation about AWS cross-account IAM use case

[123BLiN]

I spent a couple of days trying to get Grafana pod to work with multiple CloudWatch datasources as well as Prometheus to discover targets in multiple AWS accounts. So maybe this small update will save time to others.

[pingles]

I think it's good to document this kind of scenario (I seem to remember it's come up before) where people have wanted to be able to assume more than one role to a pod.

Having said that, I'm not sure I follow the example policies. Maybe it's worth documenting the example with a picture to help illustrate.

In the README there's a link to a video that @jbeda recorded and he drew a really good sketch of how roles and policies came together, using that as a starting point would be cool.