readme: add example how to delete image

[glensc]

gitlab_registry = GitLabRegistry(c['gitlab_base_url'], c['registry_base_url'], c['username'], c['access_token'])
gitlab_registry.delete_image(repository, image_hash)

how to obtain image_hash from image tag. is it possible with this library?

[IngoMeyer441]

Currently, it is not possible to query the image hash/digest of a tagged image. But it should be possible to add this feature since it is part of the image manifest.