Add step in 'Manually Install on Windows'

After cloning the repo, `git submodule` must be called inside `vimfiles`. I would add this step.

```
cd C:\Program Files\Vim
rmdir /s vimfiles
git clone git://github.com/vgod/vimrc.git vimfiles
cd vimfiles
git submodule update --init
```

For powershell users ¯\ _(ツ)_/¯, option `/s` doesn't work; it should be `rmdir <path>` or `Remove-Item -recurse <path>`
