Update README.md with vundle install direction

Without vundle installed, these errors might be seen:

    Error detected while processing /home/sgnl/.vimrc.bundles:
    line   59:
    E117: Unknown function: vundle#rc
    line   77:
    E492: Not an editor command:         Bundle 'gmarik/vundle'
    line   78:
    E492: Not an editor command:         Bundle 'MarcWeber/vim-addon-mw-utils'
    line   79:
    E492: Not an editor command:         Bundle 'tomtom/tlib_vim'
    line  104:
    E492: Not an editor command:             Bundle 'scrooloose/nerdtree'