Document how to let the lua LSP find local libraries, such as plenary

[Murderlon]

This template is great, it saved a lot of time.

There is just one thing I don't understand, which is how to make lua_ls (language server for lua) understand where to find the code for plenary. By default plenary is put into /tmp.

I tried to create a .luarc.json to find it but I failed:

{
  "$schema": "https://raw.githubusercontent.com/sumneko/vscode-lua/master/setting/schema.json",
  "diagnostics.globals": ["describe", "it"],
  "workspace.library": ["?.lua", "?/init.lua", "/tmp/plenary.nvim/lua/plenary/?.lua"]
}

Since the docs of plenary are so limited and the LSP doesn't know anything about the available methods and imports of plenary, it's quite hard to develop locally.