Improve Documentation on how to use RDK/Config in a cross-account/multi-account/Org solution

[bmorrissirromb]

The documentation in cross-account may not be the latest and greatest. At minimum, linking to some sources that go into more depth could be worthwhile, like: https://aws.amazon.com/blogs/mt/deploying-custom-aws-config-rules-in-an-aws-organization-environment/

This could be a good issue for someone who is getting ramped up into RDK.