chore(readme): update README.md for global options
chore(readme): update README.md for global options

Add an example of specifying a global config value
deepuke/ui-select2,mufl1h/ui-select2,angular-ui/ui-select2,mufl1h/ui-select2,Intilery/ui-select2,Abbaty/ui-select2,deepuke/ui-select2