docs: document spitfire option

Fixes #214

Author: mhanberg

README.md

@@ -113,6 +113,7 @@ elixir.setup {
     enable = false, -- defaults to false
     port = 9000, -- connect via TCP with the given port. mutually exclusive with `cmd`. defaults to nil
     cmd = "path/to/next-ls", -- path to the executable. mutually exclusive with `port`
+    spitfire = true, -- defaults to false
     init_options = {
       mix_env = "dev",
       mix_target = "host",