Update autocmd instructions based on new nvim api

This commit updates the README with instructions for using Neovim's native API for setting autocmds.

The README updates should address issues like terrortylor#57

README.md

@@ -131,16 +131,24 @@ vim.api.nvim_buf_set_option(0, "commentstring", "# %s")
 ```
 
 You can also use an autocommand to automatically load your `commentstring` for
-certain file types:
+certain file types. For example, to load a `commentstring` for sql files,
+in a config file, add:
 
-```vim
-" when you enter a (new) buffer
-augroup set-commentstring-ag
-autocmd!
-autocmd BufEnter *.cpp,*.h :lua vim.api.nvim_buf_set_option(0, "commentstring", "// %s")
-" when you've changed the name of a file opened in a buffer, the file type may have changed
-autocmd BufFilePost *.cpp,*.h :lua vim.api.nvim_buf_set_option(0, "commentstring", "// %s")
-augroup END
+```lua
+-- Creates an autocmd group for comment specifications
+vim.api.nvim_create_augroup("comment", { clear = true })
+
+-- Creates an autocmd that runs on BufEnter and BufFilePost
+-- We use the `BufFilePost` trigger so that we can comment after changing file extensions
+-- Without needing to repoen the buffer 
+vim.api.nvim_create_autocmd({"BufEnter", "BufFilePost"}, {
+  group = "comment",
+  pattern = {"*.sql"},
+  callback = function()
+    -- In SQL, lines are commented with "--"
+    vim.api.nvim_buf_set_option(0, "commentstring", "-- %s")
+  end
+})
 ```
 
 Or add the comment string option in the relevant `filetype` file: