added how to automatically load commentstring (#21)

* added how to automatically load commentstring

* Update README.md

Co-authored-by: Alex Tylor <alex@tylor.co.uk>

README.md

@@ -83,6 +83,26 @@ If you want to override the comment markers or add a new filetype just set the *
 vim.api.nvim_buf_set_option(0, "commentstring", "# %s")
 ```
 
+You can also use an autocommand to automatically load your **commentstring** for certain file types:
+```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")
+auground END
+```
+
+Or add the comment string option in the relevant `filetype` file:
+```vim
+let commentstring="# %s"
+```
+
+```lua
+vim.api.nvim_buf_set_option(0, "commentstring", "# %s")
+```
+
 # Installation
 
 Install just as you would a normal plugin, here are some options: