Document how to run `rcup` in customizations
Document how to run `rcup` in customizations

* thoughtbot/laptop doesn't explicitly depend on thoughtbot/dotfiles.
* So, we don't think it needs to be in the script.
* Show it as another example of customizations for those who might be
  interested in updating their machine and dotfiles in one `sh mac`.

https://github.com/thoughtbot/laptop/pull/408

iphilgood/laptop