add warning for treesitter-dart and indentation

[4e554c4c]

I thought a small addition to the readme might help reduce the number of issues (#298 #267 #266) created in this repo due to the related bug

[akinsho]

@4e554c4c tbh I think a more general warning would useful to explain indentation and highlighting don't happen hear so no issues should be raised about that sort of thing here. I've almost certainly lost perspective as an almost exclusive neovim user, but I don't know why given the contents of the README already people thing that an LSP plugin would be the cause of any of this.