travis-ci.org now displays "Please be aware travis-ci.org will be
shutting down in several weeks, with all accounts migrating to
travis-ci.com. Please stay tuned here for more information."
As GitHub actions is free for public repositories, whereas it's unclear
to what travis-ci.com will be, and the switching cost is low, it seems
going with workflows is the prudent thing to do.
Information is duplicated between the two. I think it is simpler to keep
the README focused on a Middleman user's perspective, whereas
CONTRIBUTING.md can focus on a contributor's perspective.
My motivation for this is that I'd like to later add more extensive
documentation about testing Middleman from a contributor's perspective,
but doing this in the README seemed like it would bloat it too much.