[jaymar921]

Adds a crash spin for shot down aircraft to the phase 6 (1.1.0-beta.2) checklist in docs/ROADMAP.md.

• Planes and jets roll into a nose-down spiral instead of just falling.
• Helicopters spin faster than today, with some wobble.
• The spin direction comes from the entity id, so every client sees the same thing.

This changes documentation only.