md-chips with ngMessage

Angular Material Design Documentation says that validation is pending feature so we'll just wait for that. Till then you can use this quick solution.

<md-chips ng-model="vm.offer.year">

Since this model contains an array you can check its length and use it to show validation like

<span ng-show="vm.offer.year.length == 0"> This field is required. </span>

This way you can kind of define the minimun length for chips and md-max-chips is there for max-length.

working example. http://codepen.io/next1/pen/BKzgrY