doc: add hitest notes and add termguicolors to example setups

README.md

@@ -62,6 +62,9 @@ Setup should be run in a lua file or in a lua heredoc [:help lua-heredoc](https:
 vim.g.loaded_netrw = 1
 vim.g.loaded_netrwPlugin = 1
 
+-- set termguicolors to enable highlight groups
+vim.opt.termguicolors = true
+
 -- empty setup using defaults
 require("nvim-tree").setup()