Add install instruction for plain Vim 8

[abitrolly]

[davidhalter]

I would rather not have all those different ways of installing jedi-vim. I would probably just recommend https://github.com/junegunn/vim-plug and remove the other options. @blueyed can decide.

[blueyed]

Yeah, having all the different options of installment are a recurring issue with READMEs across (Neo)Vim plugins. I think people in general should be aware how to install a plugin based on some GitHub URL/slug.

Here however it is a nuisance that submodules are used, which are not handled with every package manager, or when using no package manager at all.

I agree in general that the README section should be revisited probably, and that it should contain instructions for Vim and NeoVim, which both have support for packages (which this change is about apparently), and then maybe vim-plug as an example for a plugin manager.

[davidhalter]

@blueyed Feel free to merge whatever here.