Document the relation between fmu-drogon and fmu-drogon-flow-files

[roliveira]

We have an input (fmu-drogon) and an output (fmu-drogon-flow-files) repository for the Drogon model, but there is no explicit mention of this anywhere. Including a simple mention in the README.md of both repository will make things clearer for an external user/contributor.

[rnyb]

It is mentioned/described in the readme file of fmu-drogon-flow-files: https://github.com/equinor/fmu-drogon-flow-files?tab=readme-ov-file#drogon-flow-model-files, but we could consider also mentioning it in this repo