message: Update root README to reference sprint 4
diff: Prague is investigating and prototyping key design questions around interactive documents and MUIs.
  -We're currently in the middle of [Sprint 2](./doc/sprints/sprint2/readme.md). Highlights include rich text, git based storage, and intelligent services in an augmentation loop.
  +We're currently in the middle of [Sprint 4](./doc/sprints/sprint4/readme.md) and focused on self-hosting.
  ## Source Code Overview
  