Add extra detail around using MacVim

docs/restoring_vim_and_neovim_sessions.md

@@ -13,3 +13,7 @@
 `tmux-resurrect` will now restore vim and neovim sessions if `Session.vim` file
 is present.
 
+> If you're using the vim binary provided by MacVim.app then you'll need to set `@resurrect-processes`, for example:
+> ```
+> set -g @resurrect-processes '~Vim -> vim'
+> ```