Make the expected workflow more clear

[tgross35]

We used to have more PRs target libc-0.2. However, now that a lot of work has gone into un-diverging the branches, we basically want everything to target main and then get the backport if needed, so main is always the most up-to-date. I've been doing backports in batches and I think that's easier for everyone than all contributors cherry picking their own PRs (we used to request this but it was too conflict-prone).

So, README.md and CONTRIBUTING.md should be updated to reflect the current state of things in a more obvious way.