HTML

select placeholder

// A non-CSS - no JavaScript/jQuery answer: 
// add option and disable it, it will act like placeholder.  
<select>
    <option value="" disabled selected>Select your option</option>
    <option value="optionA">optionA</option>
</select>

select add placeholder

//The Javascript
$('select').select2({
    minimumResultsForSearch: -1,
    placeholder: function(){
        $(this).data('placeholder');
    }
});
//Add this in HTML
<select data-placeholder="Your Placeholder" multiple>
    <option></option> 
    <option>Value 1</option>
    <option>Value 2</option>
    <option>Value 3</option>
    <option>Value 4</option>
    <option>Value 5</option>
</select>