Update README.markdown

Adjusted the wording of one of the tips, and added a sentence about the type of NERDTree window that is displayed. See pull reqest #628.

README.markdown

@@ -100,14 +100,13 @@ Stick this in your vimrc:
 
 Note: Now start vim with plain `vim`, not `vim .`
 
-
 ---
-> How can I open NERDTree as a window automatically when vim starts up on opening a directory?
+> How can I open NERDTree automatically when vim starts up on opening a directory?
 
     autocmd StdinReadPre * let s:std_in=1
     autocmd VimEnter * if argc() == 1 && isdirectory(argv()[0]) && !exists("s:std_in") | exe 'NERDTree' argv()[0] | wincmd p | ene | endif
 
-This trick also prevents NERDTree from hiding when first select a file.
+This window is tab-specific, meaning it's used by all windows in the tab. This trick also prevents NERDTree from hiding when first selecting a file.
 
 ---
 > How can I map a specific key or shortcut to open NERDTree?