add co-queue example

for working with pushing sources.

``` js
function wrap(emitter, event){
var queue = new Queue;
emitter.on(event, queue.push);
return function*(end){
if (!end) return yield queue.next;
};
}

var read = wrap(emitter, 'data');
while (true) console.log(yield read());
```

Start by reviewing the repository's existing examples and documentation to find where stream usage examples belong, then compare the requested co-queue snippet with those conventions. Done means the pushing-sources example is added in the appropriate location and clearly demonstrates the shown emitter, queue, and reader flow.

Written by the indexing model from the issue text.