Publish a short public roadmap

[c0remusic]

The problem

The repository has no public roadmap. There is no place where a user can see what is being
worked on, what is planned, and what has been decided against.

Why it matters

The open issues already include several feature requests. Without a roadmap, every user who
wants a feature has to open an issue to find out whether it is already planned — and every
one of those issues has to be answered by hand.

A roadmap is also the honest place to say no: "chord export to clip is done", "MPE is
not planned", "Push 3 support depends on X". Saying no once in public is cheaper than
saying it privately five times.

Definition of done

• a short roadmap — roughly twenty lines — visible from the README, listing what is
  shipped, what is next, and what is explicitly out of scope;
• it uses the issue tracker as its source of truth (link to open issues and milestones)
  rather than duplicating a list that will go stale;
• the README links to it from the top section, where a new user will see it.

Keeping it short is the point: a roadmap nobody updates is worse than none.