doc(association-select): adding examples for placeholder options
doc(association-select): adding examples for placeholder options

Adding examples for all attributes (hidePlaceholder, selectablePlaceholder, placeholderText) used for setting the placeholder options.

Backwards compatible, and fixing issues #169 and #170

doktordirk/aurelia-orm,bas080/aurelia-orm,bas080/aurelia-orm,freska-fi/aurelia-orm,freska-fi/aurelia-orm,SpoonX/aurelia-orm,SpoonX/aurelia-orm,doktordirk/aurelia-orm