Updated Customization (markdown)

Customization.md

@@ -68,4 +68,20 @@ function! g:goyo_after()
 endfunction
 
 let g:goyo_callbacks = [function('g:goyo_before'), function('g:goyo_after')]
+```
+
+## Ensure `:q` to quit even when Goyo is active
+
+Suggested by [mm2703](https://github.com/mm2703) and [axelGschaider](https://github.com/axelGschaider) in [#16](https://github.com/junegunn/goyo.vim/issues/16)
+
+```vim
+" Quitting whether Goyo is active or not
+ca wq :w<cr>:call Quit()<cr>
+ca q :call Quit()<cr>
+function! Quit()
+  if exists('#goyo')
+    Goyo
+  endif
+  quit
+endfunction
 ```