Add group-filter attribute to repo Wiki

It would be nice to have the `group-filter` attribute added (https://github.com/angular-ui/ui-select/pull/969) to the repo's Wiki [here](https://github.com/angular-ui/ui-select/wiki/ui-select-choices). I missed it in the examples, and usually reference the wiki for this type of thing. Only stumbled across that PR because of a Google search.
