Add VIM built-in package management to the read me

[pesarkhobeee]

Dear @scrooloose
I just updated the README file in order to cover the new feature of VIM version 8 which is built-in package management. hope it brings more ease for new users of your awsome plugin :)

[alerque]

Thanks for the contribution @pesarkhobeee! The original author of this hasn't been active for many years now, I've been managing it for 3 years or so and it was stagnant for a long while before that (see #400 for current status), but yes the author did setup a great plugin.

I agree some instructions for installing manually into plain VIM 8's package path are in order, but I think the actual wording here kind of muddies the water of what a plugin manager actually does. Can you review my notes on that and update accordingly?