Add instructions for maintaining repo

[Eeems]

It would be useful to have instructions for the expected workflow of this repo.

• How to update an image
• How to create a new version of images
• How to add a new image
• etc

[Eeems]

What I've discovered so far:

• v1.x and v2.x branches are where the work is actually done
• PRs from forks fail to build until they've been merged. Use branches on this repo until this can be fixed, I guess.
• Merging a PR will update the latest tag, but not create new tags
• Creating a new GitHub release will create a new image tag as well

[Eeems]

It looks like new images are published as a new package, but it's private by default and needs to be changed to public.

[Eeems]

You will also need to update CHANGELOG.md in the main branch when a new release tag has been created.

[Eeems]

Creating a new v#.x branch requires updating the readme in the main branch with the latest information.