Document how we roadmap

[nimbinatus]

Document and maintain a public roadmap or other forward looking planning document or tracking mechanism.

We could use the milestones (e.g., https://github.com/cilium/cilium/milestones) or even a project board (e.g., Argo CD Roadmap (view)), but it probably would be good to have a ROADMAP.md with the general, high-level information on project (and subproject) direction, even if it's documenting that we use milestones.

Examples:

• https://github.com/buildpacks/community/blob/main/ROADMAP.md
• https://github.com/containerd/containerd/blob/main/ROADMAP.md
• https://lima-vm.io/docs/community/roadmap/