Added instructions for Vim's built in package manager

[iandstanley]

No description provided.

[alerque]

Umm...I think VIM (not NeoVIM) has a much simpler ways of doing this using an actual package manager, this is not using the built in "package manager" it is just doing a full manual installation. The plethora of ways to install is confusing enough without actually completely conflating this in instructions.

Additionally setting up bash functions to manage VIM plugins should be out of scope for a VIM plugin to prescribe as a management method. Lets keep this as simple as possible. Instructions for the actual VIM builtin plugin manager would be fine, and a full manual installation would be okay, or even both, but not a conflation of those things and not instructions on setting up a home-grown plugin manager.