README.md: Add example of custom validators
README.md: Add example of custom validators

sophypal/ember-frost-bunsen,sophypal/ember-frost-bunsen,sandersky/ember-frost-bunsen,ciena-frost/ember-frost-bunsen,ciena-frost/ember-frost-bunsen,ciena-frost/ember-frost-bunsen,sandersky/ember-frost-bunsen,sophypal/ember-frost-bunsen,sandersky/ember-frost-bunsen