Document extending this repo for add customized images

[BWibo]

We are open for extending this repo with other images, that address the needs of your municipality/organizations/anything else.
This fact should be documented more cleary in README.md.

• Describe the structure of the repo and why we chose this structure more clearly
• Describe the conditions we have to add you own image/folder
  • The images needs to use the base image provided in this repo
  • Proper documentation
  • Move any documentation of custom images and base image to subfolders
• Describe the process of how getting your folder with you own image
  • Add folder
  • Add docs on what extensions are included
  • Add/adapt workflow, if CI/CD features are desired