Add note in README: trick to open NERDTree as a window on startup.

[ggicci]

Update README file, add directives to open NERDTree as a window automatically when vim starts up on opening a directory. Which can prevent NERDTree from hiding when first file selected as issue #494 pointed out.

[PhilRunninger]

I don't like the term "tab window". Tabs and Windows are separate things in vim, and putting them together like this is a bit confusing. I suggest just "window".

[ggicci]

Sorry, I meant b:NERDTree._type == "tab" here. But it's really confusing for others.

[PhilRunninger]

Martin uses the term "tab tree" in #494. How do you like this sentence?

How can I open NERDTree as a tab-tree window automatically when vim starts up on opening a directory?

[PhilRunninger]

@ggicci, It looks like this PR is a continuation of #627, which you closed. Did you know you can continue committing to your branch, and the pull request will get those changes as well?

[PhilRunninger]

This is very similar to, and better than, another tip in the README. Go ahead and remove this one while you're at it.

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> How can I open NERDTree on startup, and have my cursor start in the other window?

Stick this in your vimrc:

    autocmd vimenter * NERDTree
    autocmd vimenter * wincmd p

 *via [stackoverflow/Yohann](http://stackoverflow.com/questions/4277808/nerdtree-auto-focus-to-file-when-opened-in-new-tab/19330023#19330023)*

[kevincantu]

Thank you @ggicci !!!