Make quick start documentation correct/usable

[k98kurz]

Describe the feature

The quick start documentation contains the following line in one of the configuration files:

-- This file can be loaded by calling `lua require('plugins')` from your init.vim

This is not accurate. I get only errors when I try to follow this direction. Please update to include accurate information for how to get this working.

Edit: I am using init.lua (because everything else requires it, and using both together does not work), and when I use require('plugins'), it fails with the following error:

Error detected while processing /home/username/.config/nvim/init.lua:
E5113: Error while calling lua chunk: vim/_editor.lua:0: /home/username/.config/nvim/init.lua..nvim_exec2() called at /home/username/.config/nvim/init.lua:0: Vim(packadd):E919: Directory not found in 'packpath':
 "pack/*/opt/packer.nvim"
stack traceback:
        [C]: in function 'nvim_exec2'
        vim/_editor.lua: in function 'cmd'
        /home/username/.config/nvim/lua/plugins.lua:4: in main chunk
        [C]: in function 'require'
        /home/username/.config/nvim/init.lua:541: in main chunk