Description of problem:

The issue PROJQUAY-4296 Change default mirroring behaviour has been fixed in quay 3.7.7 .So the default mirroring behaviour has been changed. It is better to have a document to descibe the change in detail, and describe the usage of the new setting `REPO_MIRROR_ROLLBACK` too. 

Details:

• Now by default if a mirror operation fails it will not rollback the state of the repo, it will instead just log the images that failed to mirror
• Rolling back can be turned back on by setting `REPO_MIRROR_ROLLBACK: true`
• There is a known issue where if a manifest list is being mirrored containing signed Docker V2 schema 1 images, the image still appears in the tag list