add CDNJS version badge in README.md
add CDNJS version badge in README.md

This badge will show the version on CDNJS!
gertot/angularjs-dropdown-multiselect,dotansimha/angularjs-dropdown-multiselect,jjwon0/angularjs-dropdown-multiselect,jjwon0/angularjs-dropdown-multiselect,dotansimha/angularjs-dropdown-multiselect,gertot/angularjs-dropdown-multiselect