please add demo for secondary-progress

[davidmaxwaterman]

There is an example in the docs, but not a demo. It would be useful to know what to expect before we try to code it.

[keanulee]

Like this one here? https://github.com/PolymerElements/paper-progress/blob/master/demo/index.html#L92

[davidmaxwaterman]

hrm, perhaps, though it would be better if it were shown animated by 'real' data (albeit simulated). I guess I don't quite 'get it', tbh.... I see in the text it gives an example of a buffer level, but if the buffer level is 10% and the main progress is 20% then you don't get to see the buffer level at all? I suppose I guess I would expect to see two different progress bars or something.