message: [docs] making it clear sqlite shouldn't be used in a cluster
diff: @@ -498,6 +498,11 @@ look something like:
  RESULTS_BACKEND = RedisCache(
  host='localhost', port=6379, key_prefix='superset_results')
  +Note that it's important that all the worker nodes and web servers in
  +the Superset cluster share a common metadata database.
  +This means that SQLite will not work in this context since it has
  +limited support for concurrency and
  +typically lives on the local file system.
  Also note that SQL Lab supports Jinja templating in queries, and that it's
  possible to overload
  