Updated Customization (markdown)

Customization.md

@@ -1,3 +1,29 @@
+## Callbacks for GVim
+
+```vim
+function! g:goyo_before()
+  if has('gui_running')
+    set fullscreen
+    set background=light
+    set linespace=7
+  elseif exists('$TMUX')
+    silent !tmux set status off
+  endif
+endfunction
+
+function! g:goyo_after()
+  if has('gui_running')
+    set nofullscreen
+    set background=dark
+    set linespace=0
+  elseif exists('$TMUX')
+    silent !tmux set status on
+  endif
+endfunction
+
+let g:goyo_callbacks = [function('g:goyo_before'), function('g:goyo_after')]
+```
+
 ## Disabling plugins
 
 ### MiniBufExpl with `g:miniBufExplBuffersNeeded` set