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`.

thoughtbot#408

README.md

@@ -141,6 +141,11 @@ For example:
 brew_install_or_upgrade 'go'
 brew_install_or_upgrade 'ngrok'
 brew_install_or_upgrade 'watch'
+
+if [ -r "$HOME/.rcrc" ]; then
+  fancy_echo "Updating dotfiles ..."
+  rcup
+fi
 ```
 
 Write your customizations such that they can be run safely more than once.