message: add info about playback and synthetic boards
diff: @@ -7,6 +7,12 @@ using json = nlohmann::json;
  // clang-format off
  json brainflow_boards_json = {
  {"boards", {
  + {"-3",
  + {{"name", "PlayBack"}
  + }},
  + {"-2",
  + {{"name", "Streaming"}
  + }},
  {"-1",
  {{"name", "Synthetic"},
  {"sampling_rate", 250},
  