I remember adding that message a long time ago so the user has a trace
of working copy commit ids in the terminal output. They should be able
to get the same information from the operation log combined with
e.g. `jj st --at-op`.
The project is starting to work well enough that I think it's time
that the documentation targets users instead of VCS hackers. This
patch rewrites much of the README to describe how the features help
the user instead of describing how they work. It also adds a tutorial.
