Document how to override exclude-packages

[realnedsanders]

I'd like to build fcos with python3 installed so that the system is able to be managed via ansible.
I'm following the established pattern of including this repo as a git submodule in my own custom-coreos-config repo.

Python3 is listed under exclude-packages in manifests/fedora-coreos.yaml.
Currently the only way I can see to achieve my goal is to modify manifests/fedora-coreos.yaml, or to maintain my own manifest.yaml and manifests/fedora-coreos.yaml.

There has to be a better way, I'm just not able to find the documentation to support it.