Vim – helptags不适用于NERDtree

我下载并手动安装NERDTree
http://www.vim.org/scripts/script.php?script_id=1658

@hits ➜  .vim rvm:(-ruby-1.9.2)  ls -laR
.:
total 28
drwxr-xr-x  6 hitsu hitsu 4096 2012-02-16 15:21 .
drwxr-xr-x 49 hitsu hitsu 4096 2012-02-27 17:43 ..
drwxrwxr-x  2 hitsu hitsu 4096 2011-12-28 14:18 doc
drwxrwxr-x  2 hitsu hitsu 4096 2011-12-28 14:17 nerdtree_plugin
-rw-rw-r--  1 hitsu hitsu  283 2012-02-07 11:05 .netrwhist
drwxrwxr-x  2 hitsu hitsu 4096 2011-12-28 14:17 plugin
drwxrwxr-x  2 hitsu hitsu 4096 2011-12-28 14:17 syntax

./doc:
total 60
drwxrwxr-x 2 hitsu hitsu  4096 2011-12-28 14:18 .
drwxr-xr-x 6 hitsu hitsu  4096 2012-02-16 15:21 ..
-rw-rw-r-- 1 hitsu hitsu 52744 2011-12-28 14:17 NERD_tree.txt

./nerdtree_plugin:
total 20
drwxrwxr-x 2 hitsu hitsu 4096 2011-12-28 14:17 .
drwxr-xr-x 6 hitsu hitsu 4096 2012-02-16 15:21 ..
-rw-rw-r-- 1 hitsu hitsu 1614 2011-12-28 14:17 exec_menuitem.vim
-rw-rw-r-- 1 hitsu hitsu 8124 2011-12-28 14:17 fs_menu.vim

./plugin:
total 132
drwxrwxr-x 2 hitsu hitsu   4096 2011-12-28 14:17 .
drwxr-xr-x 6 hitsu hitsu   4096 2012-02-16 15:21 ..
-rw-rw-r-- 1 hitsu hitsu 126233 2011-12-28 14:17 NERD_tree.vim

./syntax:
total 12
drwxrwxr-x 2 hitsu hitsu 4096 2011-12-28 14:17 .
drwxr-xr-x 6 hitsu hitsu 4096 2012-02-16 15:21 ..
-rw-rw-r-- 1 hitsu hitsu 3741 2011-12-28 14:17 nerdtree.vim

我试图在vim中打开NERDtree的帮助文件.
但是当我键入：helptags我得到错误E471：需要保留

之后,我键入：help NERD_tree.txt但我得到错误E149：对不起,找不到NERD_tree.txt的帮助

帮助说明取自：
http://www.vim.org/scripts/script.php?script_id=1658

我知道我可以在任何编辑器中手动打开NERD_tree.txt(例如vim或Emacs),但我只是出于好奇心.

PS我是一个vim noob. ：P

您必须给出：helptags帮助文件(您要解析的)的目录是.例如：

:helptags ~/.vim/doc