doc: setup call is only allowed once

README.md

@@ -33,8 +33,11 @@ use {
 ## Setup
 
 Setup should be run in a lua file or in a lua heredoc (`:help lua-heredoc`) if using in a vim file.
+
 Legacy `g:` options have been migrated to the setup function. See [this issue](https://github.com/kyazdani42/nvim-tree.lua/issues/674) for information on migrating your configuration.
 
+Setup may only be run once; subsequent calls will result in a warning. Do not invoke from, say, a packer config function and then call it again later.
+
 ```vim
 " vimrc
 nnoremap <C-n> :NvimTreeToggle<CR>