Add installation steps via vim-plug

[zamirmf]

Link to vim-plug.

.vimrc extract:

 18 let g:ackprg = 'ag --vimgrep'
 19 
 20 call plug#begin()
 21 Plug 'junegunn/fzf', { 'do': { -> fzf#install() } }
 22 Plug 'mileszs/ack.vim'
 23 call plug#end()

:PlugStatus output:

  1 Finished. 0 error(s).                                                                                           
  2 [==]                                                                                                                  
  3                                                                                                                   
  4 - fzf: OK                                                                                                            
  5 - ack.vim: OK

:Ack usage: