Fix README autocmd suggestion (#1330)

Author: nickspoons

README.markdown

@@ -178,7 +178,7 @@ autocmd BufEnter * if bufname('#') =~ 'NERD_tree_\d\+' && bufname('%') !~ 'NERD_
 
 ```vim
 " Open the existing NERDTree on each new tab.
-autocmd BufWinEnter * if getcmdwintype() == '' | silent NERDTreeMirror | endif
+autocmd BufWinEnter * if &buftype != 'quickfix' && getcmdwintype() == '' | silent NERDTreeMirror | endif
 ```
 or change your NERDTree-launching shortcut key like so:
 ```vim