message: docs: clarify legacy vs non-legacy in README
diff: These packages are extracted with minimal changes (almost as-is). They also depend on legacy API (`viz.py`) to function.
  `@superset-ui/plugin-*` packages are newer and has higher quality in general.
  +A key difference that they do not depend on `viz.py`
  +(which contain visualization-specific python code) and interface with
  +`/api/v1/query/` instead: a new generic endpoint instead meant to
  +serve all visualizations.
  We are not accepting pull requests for new community-contributed plugins to be merged into this repository at the moment.
  We will keep it lean for now to improve the standard and reduce operational load on maintenance.
  