feat: Fix markdown provider icon kind

Closes #2

lua/outline/providers/init.lua

@@ -3,6 +3,11 @@ local M = {}
 local providers = {
   'outline/providers/nvim-lsp',
   'outline/providers/coc',
+  -- NOTE: There is in fact a markdown LSP that can provide symbols. However
+  -- on buffer open the LSP may not be attached immediately. Before the LSP is
+  -- ready if the user opens the outline, our own markdown provider will be
+  -- used. After refreshing/reopening, the provider will then switch to the LSP
+  -- (if the user has a markdown LSP).
   'outline/providers/markdown',
 }