docs: update FAQ snippets containing quit command. (#1417)

Author: rzvxa

README.markdown

@@ -150,6 +150,24 @@ autocmd VimEnter * if argc() == 1 && isdirectory(argv()[0]) && !exists('s:std_in
 
 ### How can I close Vim or a tab automatically when NERDTree is the last window?
 
+Because of the changes in how Vim handles its `autocmd` and layout locking `quit` command is no longer available in Vim9 auto commands, Depending on which version you're running select one of these solutions.
+
+__NeoVim users should be able to choose either one of them!__
+
+#### Vim9
+
+```vim
+" Exit Vim if NERDTree is the only window remaining in the only tab.
+autocmd BufEnter * if tabpagenr('$') == 1 && winnr('$') == 1 && exists('b:NERDTree') && b:NERDTree.isTabTree() | call feedkeys(":quit\<CR>:\<BS>") | endif
+```
+---
+```vim
+" Close the tab if NERDTree is the only window remaining in it.
+autocmd BufEnter * if winnr('$') == 1 && exists('b:NERDTree') && b:NERDTree.isTabTree() | call feedkeys(":quit\<CR>:\<BS>") | endif
+```
+
+#### Vim8 or older
+
 ```vim
 " Exit Vim if NERDTree is the only window remaining in the only tab.
 autocmd BufEnter * if tabpagenr('$') == 1 && winnr('$') == 1 && exists('b:NERDTree') && b:NERDTree.isTabTree() | quit | endif