Added Plug installation option to README.md

[sTiKyt]

No description provided.

[Cimbali]

This is interesting, thanks @sTiKyt but I think the first option should be :packadd since vim 8 has integrated package management. We can also probably remove the ~/.vim option.

[sTiKyt]

Welp, been 4 years since this PR, I don't even remember what it was about so it's up to yall to decide what to do with it, I still think it would help 😅