message: docs: A few cleanups to SharedTree roadmap
  Makes links relative, clearer linkage in readme.
diff: # @fluid-internal/tree
  -This DDS is not yet ready for public consumption. See [roadmap.md](docs/roadmap.md).
  +This DDS is not yet ready for public consumption. For a high-level overview of the goals of this project, see the [roadmap](docs/roadmap.md).
  ## Motivation
  