docs(dimmer): Add example of Loader

[edcarroll]

Based off of conversation in #353, it might be worth adding a simple example of a loader component in the dimmer docs:

<sui-dimmer class="inverted" [isDimmed]="isLoading" [isClickable]="false" [transitionDuration]="200">
    <div class="ui text loader">Loading</div>
</sui-dimmer>