should document how to re-review changes from imports

[Darragh Bailey]

Git upstream should include some documentation on how to support re-reviewing changes from an import in order to allow catching of issues should someone modify one of the carried changes.

It seems a common use case is to simply push up the import branch to the remote, and only request the CI tooling to recheck the import merge. This is a great way for unintended changes to the carried patches to bypass review, not to mention has the unfortunate consequence of making it more difficult to amend changes should the merge fail to pass the CI tests due to bugs introduced between local changes and upstream.

This is a combination of a documentation of a use-case and addition of some helper job descriptions to the contrib/ folder to allow users to build workflows around the tool by using well defined examples.